The AF error _might_ depend on:
3. Focal Length (if zoom lens)
4. Chosen AF point
5. Defocus method (is the focus moving in one or the other direction)
6. Temperature, moon phase and whatnot...
Further, you want to have some confidence in the readings (are your results just a random accident, or is it repeatable?)
If you are going to test all of these dimensions manually, you are going to have your hands full. I have been using Reikan Focal that automates the process to a large degree, I still find it to be tedious and "unpredictable" (if I find that one lens performs best at +4 in one test, then do a similar test 1 month later finding that it works best at -2, what does this say about the relevance of the entire exercise? What did I gain in terms of IQ during that month by using +4?)