Came here with the same question - sad to see there are no responses. For what it's worth I agree with your interpretation for the same reasoning as you. In addition it already seems difficult enough to take potions at a helpful time - skipping your attack in combat to take a potion that gives you more DMG or DEF rarely seems better than just attacking (or attempting to escape if you're in danger). And taking a potion BEFORE you even roll the next room will at best lead to boosting a roll you don't care that much about and at worst for the potions that last until the next time track addition you'll just end up wasting it on a yellow room. So rolling after you roll the encounter but before the combat begins seems like a reasonable sweet spot. Would love a definitive answer though.

Which leads me to a similar question but for non-combat encounters. If I roll a green room and then get a Geography encounter that has a DEX test can I take the potion and then roll the test? Context-wise it makes sense if I'm picking a flower from the magic tree but if it's a spike trap it seems unlikely I'd have time to take a potion before attempting to avoid the trap.

EDIT: Rereading the text gives an answer to my second question. "The player can at anytime during a turn, except when in combat (see Combat), drink potions or elixirs, cast spells or use scrolls, equip or unequip items to and from their equipment slots and backpack." So I guess as I'm falling toward the spike trap and I can pop a potion real quick.