WARNING: Totally unnecessary geek post ahead. Stop now if you're more interested in photography than maths... (and wild guesswork).
I've been experimenting with a 5D4 and some lenses, using a test chart that will show where the focus plane is (in terms of centimetres in front/behind the intended plane). I was wondering how the results might relate to MFA values. The only reference to the "size" of one Canon MFA unit that I've been able to find is that 1 unit roughly equates to 1/8th of the total DOF for the len's focal length & subject distance when shot wide open.
200mm at f/2.8 and 3.5m camera-to-subject distance is about 5.2cm DOF. One MFA unit should then be 5.2/8=0.65cm. The camera was consistently 1.5cm back focusing, so I make that 1.5 / 0.65 = 2.3 MFA units; which would need to be -2.3 to pull a back focusing lens frontwards.
The 1/8th data seems a bit coarse to me, as a standard MFA distance of 50x the focal length (3.5m for a 70mm shot), at f/2.8, has a DOF of 44cm; meaning one MFA unit is a 5.5cm shift. As that lens was back focusing by 3cm, it would indicate it needs an MFA adjustment of 3/5.5=0.54.
Has anyone got any better/different data on the "size" of a Canon MFA unit? I guess I could set up a tripod and run some tests in the absence of a definitive answer.