We recommend a micrometer instead of a caliper for the following steps. Calipers are not precise enough to measure such small values and a micrometer has a ratchet system made to apply same pressure for all measures.

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

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

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

Print settings: 0.20mm QUALITY

Filament: choose the filament profile corresponding to the filament you want to calibrate.

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

Set your extrusion multiplier to 1.00

Set the following print settings:

Spiral vase mode (and accept dialog to auto-configure settings accordingly)

Solid layers Top and Bottom: 0

Slice the model and start the print.

Measure thickness of all the perimeters and calculate the average.

Calculate your new extrusion multiplier using the formula on the 1st image. Here is an example:

Your original extrusion multiplier = 1.00 and your average perimeter thickness = 0.47mm.

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

Your new extrusion multiplier = (1.00 · 0.45)/0.47 = 0.957

Enter the new extrusion multiplier value in the filament settings page of PrusaSlicer.

Slice and print the calibration model again with the new extrusion multiplier value.

Verify the wall thickness with your micrometer. Repeat the previous steps if refinement is needed.

You can now more finely recalibrate your first layer.