the Bentley manual gives ride heights for all models , sport , non sport , rear air , full air , full springs .
they and BMW measure from the lowest point on the wheel rim to the wheel arch through the hub centre line . I do not know if the rim lip would vary on replica wheels .
I suppose the method you suggest using is ok for anyone doing similar comparison .

for example , my 2002 sport measures approximately 30 inches from the lower rim to arch on 20 inch rims so from hub centre to arch will be 20 inches if I subtract half the rim size .
I am only posting this to try to avoid any confusion to anyone reading this thread .