How to label the perimeter percentage of a concave_hull relative to another polygon layer

Question

From a layer of points 'POINTS_LAYER', I create the concave_hull using the following expression:

smooth(simplify(buffer(concave_hull(
            collect(
                $geometry,
                group_by:=overlay_nearest(@layer, "class"))
                ,0.0),0.75),2.0),5)

I add the screenshot with the result:

I am labeling the perimeter with this expression:

 format_number(round(perimeter(smooth(simplify(buffer(concave_hull(
            collect(
                $geometry,
                group_by:=overlay_nearest(@layer, "class"))
                ,0.0),0.75),2.0),5)),1)) || ' m'

I would like to label with a dynamic expression the calculated perimeter in percentage relative to the perimeter of the aggregated polygonal layer 'POLY_LAYER'. For example, if the layer 'POLY_LAYER' has a perimeter of 346.000 m, and the concave_hull created has a perimeter of 104.948 m, I would like to label the calculated percentage that represents the perimeter of the concave_hull with regards to the polygonal layer (30.06%) as you can see in this simulation:

Answer 1

If there is always just one feature in the 'POLY_LAYER' then you can use an expression like:

with_variable('points_perimeter', 
    perimeter(smooth(simplify(buffer(concave_hull(
            collect(
                $geometry,
                group_by:=overlay_nearest(@layer, "class"))
                ,0.0),0.75),2.0),5)),
with_variable('poly_perimeter',perimeter(geometry(get_feature('POLY_LAYER','id',33))),
round(@points_perimeter,1) || ' m (' || round(@points_perimeter / @poly_perimeter * 100,2) || '%)'))

Just replace the id and value in the get_feature expression with a unique field and value for your layer.

If there can be multiple features in the 'POLY_LAYER' then that expression will need to be enhanced.