OOC:
Just a reminder that 5e by default does not have facing rules. Facing is a variant rule in the DMG, so by default everyone is looking in all directions at some point each turn. Here is how the aversion option is worded in her stat block:
[ +- ] Petrifying Gaze Aversion
Unless surprised, a creature can avert its eyes to avoid the saving throw at the start of its turn. If the creature does so, it can’t see Hythonia until the start of its next turn, when it can avert its eyes again. If the creature looks at Hythonia in the meantime, it must immediately make the save.
If you look up after you get 30 feet away and don't keep averting, and she gets within 30 feet later, since she was 30 feet away or less when you started your turn, you would have looked at her in the meantime. It is your risk to take.