It can screw up calibration inside AH if you don't return to center before clicking the "OK" button.
Well yes, but ... for trim, it's not going to matter a hill of beans as long as you are somewhat close to center.
What the center point does is change how much scaling is applied to the range on either side of center. For example, if your joystick actually travels the range of 100 to 1100, and and you "center" on 600, the travel will be exactly equal for each step of change above and below 600. At 1100 AH is going to be at 65535, and at 100 AH is going to be at 0, while at 600, it's going to be at 32767. Each step of movement on the controller is going to move AH by about 65 points, throughout the entire range.
If you center on 200 (an extreme example) then at 1100 AH is still going to be at 65535, and at 100 AH is still going to be at 0 - no change at the extremes. But at 200 it's going to report 32767, and each step of movement above 200 is going to move AH up by about 37, while each step of movement below 200 is going to move AH downward from 32767 by about 3277 points. Or in other words it will move from full to the bad centerpoint slowly, and then move very quickly from center to 0 after you cross the center point.
But it's trim, not a joystick input, so as long as you are even halfway close to the center of the travel when you center, you're fine. You'll never be able to even tell the slight difference between the scaling above versus below the centerpoint as long as you look at the numbers and eyeball a place roughly half way through the calibibration range to set to before clicking OK.
And if you really want to get wiggy - go modify the files directly, and replace the center value by hand - it's the easiest way to "exactly" center an analog device that doesn't actually have a center.
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