The sweet spot for much of the music I prepare is around 7.5 mm. For studio sessions and other instances where music is sight-read, the stands are shared, and/or lighting is sub-optimal, 7.7 mm is nice; for chamber music that is likely to have the benefit of a lot of rehearsal, 7.3 mm or even smaller can be just fine.
Just finished another arrangement and these ideas are certainly useful. I usually stick to the default of 7.0 mm, but will give a slightly larger size a try. Either though feels a bit small on iPads. At least on the 9.5 inch models. I have yet to see music on one of the larger ones.