How To Adjust Aperture in FoCal Pro
Within FoCal Pro under Settings > Preferences > Pro (tab)
Once changed the new aperture value will be used for focus calibration and other tests within FoCal.
Note it is only possible to adjust Test Aperture when the camera and lens are connected to FoCal Pro.
By default FoCal will run tests and calibrations with the lens wide open (widest aperture). At widest aperture the depth of field is most shallow and focus calibration will have most impact.
If you always or almost always tend to shoot a lens stopped down then changing Test Aperture to the aperture value used in real world shooting may be useful.