A164: RAW I see this as correct, as long as the 5-foot diagonal applies vertically, which I see no reason that it would not, again, RAW. You can indeed use your 30ft flying speed to gain 30 feet of height and 30 feet of horizontal displacement at the same time if using square 5-ft "cubes" on a "grid".

RAI I would never allow such (even in AL), but RAW, I believe you're in the clear. Be prepared for your DM to say "I agree RAW, no you can't do it, this is an exception I'm making."