1) change only_one_perimeter_top to respect external_infill_margin. Old behavior can be reproduced by setting external_infill_margin to 0

2) change first_layer_height from % of layer_height to % of the diameter of the nozzle of the first extruder. It's safer and more logical that way.

src/libslic3r/Flow.cpp

@@ -132,12 +132,16 @@ Flow support_material_flow(const PrintObject *object, float layer_height)
 Flow support_material_1st_layer_flow(const PrintObject *object, float layer_height)
 {
     const auto &width = (object->print()->config().first_layer_extrusion_width.value > 0) ? object->print()->config().first_layer_extrusion_width : object->config().support_material_extrusion_width;
+    float slice_height = layer_height;
+    if (layer_height <= 0.f && !object->print()->config().nozzle_diameter.empty()){
+        slice_height = (float)(object->config().first_layer_height.get_abs_value(object->print()->config().nozzle_diameter.get_at(0)));
+    }
     return Flow::new_from_config_width(
         frSupportMaterial,
         // The width parameter accepted by new_from_config_width is of type ConfigOptionFloatOrPercent, the Flow class takes care of the percent to value substitution.
         (width.value > 0) ? width : object->config().extrusion_width,
-        float(object->print()->config().nozzle_diameter.get_at(object->config().support_material_extruder-1)),
-        (layer_height > 0.f) ? layer_height : float(object->config().first_layer_height.get_abs_value(object->config().layer_height.value)),
+        float(object->print()->config().nozzle_diameter.get_at(object->config().support_material_extruder - 1)),
+        slice_height,
         // bridge_flow_ratio
         0.f);
 }

src/libslic3r/GCode.cpp

@@ -948,7 +948,7 @@ void GCode::_do_export(Print &print, FILE *file)
     // Write some terse information on the slicing parameters.
     const PrintObject *first_object         = print.objects().front();
     const double       layer_height         = first_object->config().layer_height.value;
-    const double       first_layer_height   = first_object->config().first_layer_height.get_abs_value(layer_height);
+    const double       first_layer_height   = first_object->config().first_layer_height.get_abs_value(m_config.nozzle_diameter.empty()?0.:m_config.nozzle_diameter.get_at(0));
     for (const PrintRegion* region : print.regions()) {
         _write_format(file, "; external perimeters extrusion width = %.2fmm\n", region->flow(frExternalPerimeter, layer_height, false, false, -1., *first_object).width);
         _write_format(file, "; perimeters extrusion width = %.2fmm\n",          region->flow(frPerimeter,         layer_height, false, false, -1., *first_object).width);
@@ -2642,7 +2642,11 @@ std::string GCode::extrude_loop(const ExtrusionLoop &original_loop, const std::s
     //no-seam code path redirect
     if (original_loop.role() == ExtrusionRole::erExternalPerimeter && this->m_config.external_perimeters_vase && !this->m_config.spiral_vase 
         //but not for the first layer
-        && this->m_layer->id() > 0) {
+        && this->m_layer->id() > 0
+        //exclude if min_layer_height * 2 > layer_height (increase from 2 to 3 because it's working but uses in-between)
+        && this->m_layer->height < EXTRUDER_CONFIG(min_layer_height) * 3
+        ) {
+
         return extrude_loop_vase(original_loop, description, speed, lower_layer_edge_grid);
     }
 

src/libslic3r/PerimeterGenerator.cpp

@@ -509,7 +509,9 @@ void PerimeterGenerator::process()
                 //store surface for top infill if only_one_perimeter_top
                 if(i==0 && config->only_one_perimeter_top && this->upper_slices != NULL){
                     //split the polygons with top/not_top
-                    ExPolygons upper_polygons(this->upper_slices->expolygons);
+                    coord_t offset_top_surface = scale_(config->external_infill_margin.get_abs_value(
+                        config->perimeters == 0 ? 0 : (ext_perimeter_width + perimeter_spacing * (config->perimeters - 1))));
+                    ExPolygons upper_polygons = offset_ex(this->upper_slices->expolygons, offset_top_surface);
                     ExPolygons top_polygons = diff_ex(last, (upper_polygons), true);
                     ExPolygons inner_polygons = diff_ex(last, top_polygons, true);
                     // increase a bit the inner space to fill the frontier between last and stored.

src/libslic3r/PrintConfig.cpp

@@ -645,7 +645,11 @@ void PrintConfigDef::init_fff_params()
     def->label = L("in vase mode (no seam)");
     def->full_label = L("ExternalPerimeter in vase mode");
     def->category = OptionCategory::perimeter;
-    def->tooltip = L("Print contour perimeters in two circle, in a contiunous way, like for a vase mode. It needs the external_perimeters_first parameter do work.");
+    def->tooltip = L("Print contour perimeters in two circle, in a contiunous way, like for a vase mode. It needs the external_perimeters_first parameter do work."
+        " \nDoesn't work for the first layer, as it may damage the bed overwise."
+        " \nNote that It will use min_layer_height from your hardware setting as the base height (it doesn't start at 0)"
+        ", so be sure to put here the lowest value your printer can handle."
+        " if it's not lower than two times the current layer height, it falls back to the normal algorithm, as there are not enough room to do two loops.");
     def->mode = comExpert;
     def->set_default_value(new ConfigOptionBool(false));
 
@@ -1267,13 +1271,13 @@ void PrintConfigDef::init_fff_params()
     def->category = OptionCategory::perimeter;
     def->tooltip = L("When printing with very low layer heights, you might still want to print a thicker "
                    "bottom layer to improve adhesion and tolerance for non perfect build plates. "
-                   "This can be expressed as an absolute value or as a percentage (for example: 150%) "
-                   "over the default layer height.");
+                   "This can be expressed as an absolute value or as a percentage (for example: 75%) "
+                   "over the default nozzle width.");
     def->sidetext = L("mm or %");
     def->ratio_over = "layer_height";
     def->min = 0;
     def->mode = comAdvanced;
-    def->set_default_value(new ConfigOptionFloatOrPercent(0.2, false));
+    def->set_default_value(new ConfigOptionFloatOrPercent(75, true));
 
     def = this->add("first_layer_speed", coFloatOrPercent);
     def->label = L("Default");

src/libslic3r/Slicing.cpp

@@ -47,9 +47,11 @@ SlicingParameters SlicingParameters::create_from_config(
 	coordf_t				 object_height,
 	const std::vector<unsigned int> &object_extruders)
 {
-    coordf_t first_layer_height                      = (object_config.first_layer_height.get_abs_value(object_config.layer_height.value) <= 0) ?
+    //first layer height is got from the first_layer_height setting unless the value was garbage.
+    // if the first_layer_height setting depends of the nozzle width, use the first one.
+    coordf_t first_layer_height = (object_config.first_layer_height.get_abs_value(print_config.nozzle_diameter.empty() ? 0. : print_config.nozzle_diameter.get_at(0)) <= 0) ?
         object_config.layer_height.value : 
-        object_config.first_layer_height.get_abs_value(object_config.layer_height.value);
+        object_config.first_layer_height.get_abs_value(print_config.nozzle_diameter.get_at(0));
     // If object_config.support_material_extruder == 0 resp. object_config.support_material_interface_extruder == 0,
     // print_config.nozzle_diameter.get_at(size_t(-1)) returns the 0th nozzle diameter,
     // which is consistent with the requirement that if support_material_extruder == 0 resp. support_material_interface_extruder == 0,