I know using trim to center up the horn leaves you unequal side-to-side, so I thought that initial centering of the servo with sub-trim was to keep that inequality from happening. I know when I build a car I center it with the sub-trim as close as I can get it, align to zero toe with my Niftech blocks and drop it on the track. It NEVER requires more than one click of the external trim to track straight and true and I find my circles are equal as can be both directions. And sometimes (usually) I have to add a lot more sub-trim than I'd really like to, but that just seems to be how it is. I wonder if the 949 or 9650 have that centering ability Brian describes--that would be TOO cool.