Firstly ensure your Z Offset is dialled-in. Too much squish at the bottom may translate to too much squish at height with an initial 100% Flow Rate, which will force material into too wide a line and skewer line width measurements.

Anyway, measure the walls of your wall test from the middle because this is where your extruder will be up-to speed. Measure the top 5-10 layers of each point. Don't press too hard on the calipers but ensure you're being firm (really, this is a mistake I often witness first hand: too light a push will measure too much directional variance, since I doubt your walls are perfectly smooth vertically, and too firm will force the calipers into the material). Now run your calculations.

Finally take an average measure of your filament width over a meter of length, at least; sample at least 5 points. Enter the average filament diameter in the appropriate setting of your slicer.