Predicted best AFMA -534

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I'm trying to do some AFMA on my lenses and so have downloaded FoCal for OS X. When I run this, I have to use manual mode which involves taking a series of pictures (3 sets of 3 from -5, 0 and +5).

When I get the tool to analyse the files it predicts AFMA at -534 on my 35L on a 5D Mark iii.

Would this indicate a bad copy of this lens? It doesn't have a filter on it.

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Think I may have solved it (partially). I had the lens set on f11. When i dialled it down to 1.4 and did the same test, it came back with prediction of 2. I did the test again with AFMA set to -2, 0 and 2 and it predicted 1. I set the focus point to just to the left off the centre point and did the test again. It predicted 4. Confused. I changed the aperture to 2.8 and did the test again, it predicted -4.

Is this normal behaviour?