Measure your filament diameter with a micrometer. Write it on a piece of paper.

Download the calibration model from thingiverse.com/thing:3405991 or from github.com/gregsaun/.../test/cube

Start Prusa Slicer and load the calibration model. On the main page, select the following parameters:

Print settings: 0.20mm Normal

Filament: Prusa PLA

Enter your filament diameter into the corresponding input under filament settings.

Reset "Extrusion multiplier" to 0.95.

Set the following print settings:

Vase mode

Solid layers top and bottom: 0

Slice and print your cube.

Measure all the perimeters of your cube and calculate the average.

Calculate your new extrusion multiplier using the formula on the second image. Here is an example:

Your old extrusion multiplier = 0.95 and your average perimeter thickness = 0.43mm.

Your are using a 0.4mm nozzle, so your extrusion width is 0.45mm.

Your new extrusion multiplier = (0.95 · 0.45)/0.43 = 0.994.

Enter the new extrusion multiplier value in the filament settings page of Prusa Slicer

Print the calibration model again with these new settings.

Verify the wall thickness with your micrometer. Repeat the previous steps one or two more times if refinement is needed.

You can now more finely recalibrate your first layer. You could print the first layer of the previous calibration model as a test.