Simple answer: If altissimo is hard, open less until it becomes easier. If you play front F for alternate E a lot, then open enough to make it in tune. Personally I play altissimo a lot more than alternate E, so mine's optimized for altissimo. The opening is different on every horn. On one of my horns, it opens just a crack. On others, a couple of mm's. You'll know it's too wide if altissimo is nearly impossible.