Anyway, I have been trying this Dot Tune method today with my 400mm f/5.6.
A real disaster! At f/5.6 I got -8 and +20 MFA as limits. I was curious and tried at f/11, results -6 +16. F/32? again -8 +20. There is a high variability on the limits.
I certainly won't take the median here, because
1.this lens was fine at 0 MFA
2. the MFA does NOT modify the way this lens focuses!
Yes, you read it right: no visible effect on this lens focus, whatever MFA setting I chose! What is happening here? Very strange. I had to tune one of my Sigma lens before, and I could see the difference, for sure! But here this method and the 7D MFA settings don't seem to change anything! I wonder what went wrong?
I had only the 400 and some manual focus lens, but I will try this Dot Tune method again with the Sigma, when I have some time to go out.