I'm sure it's easy to see the difference between 0 and +10 AFMA on the ISO12233 chart. But have you tried comparing +9 vs +10 points? Not so easy I bet.
As Teamspeed points out, +/-1 changes are so small that even at f/1.2 you'll be had pressed to notice a difference. Also, PDAF is inherently inaccurate. Rarely with the lens focus to exactly the same. It gets really close, close enough that you will never see the difference in normal shooting, but there are tolerances within the mechanics of the lens.
So as you have already determined that optimal focus is somewhere from -1 to -5, split the difference.