Thanks Snake. So once I establish a MA value for infinity, should I load that to the lens through the dock before testing other distances?
i'm confused by the implications of your statement... but just to clarify here, your canon body's units of measurements of MA is not the same as the dock's software's. so, if you set +5 on your body, that is not the same as +5 on the dock. so, in regards to your statement, don't touch the body's MA at all--it should be 0 at all times. you set in the dock first then test to see if your guess is correct.
if i were you, to avoid the possibility of adjustments of one distance range affecting others, i would go in order from closest or farthest, setting all the unadjusted values to the same as what you just adjusted. in other words i'd do this:
0. make sure your body's MA is set to 0 for wide and tele if supported.
1. start 0-0-0-0 on dock
2. then set arbitrarily to 5-5-5-5.
3. retest and readjust as needed. say arriving at 7-7-7-7.
4. then repeat with the next range: 7-3-3-3.
5. then 7-3-0-0.
6. then 7-3-0-1.
7. then retest all ranges over again.
ideally, you should do this in lighting similar to what you'd typically encounter. just fyi, some of my canon lenses also change focus depending on lighting, especially big aperture primes. so it's not just a sigma thing. i think it's just how canon's af works. also, it appears full frame cameras like the 5d3 have more problems with outside points--probably due to lens distortions being more significant the farther from center.