There is no "line" or height requirement. Umpire's judgment. One possibility might be to remove your mask, clear the catcher and plate area and if you still have time to point at it and call it prior to the catch attempt it it's probably high enough. Otherwise let the chips fall. You also might want to reread the infield fly rule 2-30 page 26, 8-2-9 and 8-2-10 page 63 and reconsider your two out IFF and "let's it fall to the ground" rulings. But thats just me. We do like outs.
Paul