Brighty,
I've never tried the 90grit. Used 200 and 400gr and it worked great. Did two 4x2 trans arkies in about 20 mins each. Of course, both were already flat "in theory", but I did chamfer the edges with no issue. Maybe it's like it is with the sandpaper that finer grit is simply more effective. One person told me it's because more surface area is covered with higher grits, but I frankly don't know. If you have some higher grit, give it a whirl and let us know if it makes a difference.

Anyway you cut it, lapping an arkie is as much fun as a first marriage.