On the test for "differential or not" I quite agree with Charlie P.!
I've read more wacky tests for simple trimming than I ever could have believed. Many of them attributed to an organization or individual who should know (better).

If you adjust for best possible axial roll experience-----you've got it pure and simple!

I came from the time when we didn't have in-flight trims (save elevator) and we trimmed as you do a free-flight----without the turn of course.
Takes about 50 flights to get it "right" but once you are there it is a thing of beauty as it doesn't give you a 'different look' each day.

If yours requires some trim touch up each time you go out you simply aren't trimmed period.