It's tied to heat and oxygen specifically, as I recall. Might be wrong about oxygen, but I am sure it's individual values, not overall terraforming index.

Um. I didn't mention overall TI index. I mentioned heat specifically. The door still isn't open and I'm at 12.6 mK...more than an order of magnitude over what it says I need

Aye, did you walk up to the door or are you just going by what the console says? I think I got the same warning when I approached the console so I returned to my outpost to get the reading. I was well above the 1.0mK so I go puttering back and approach the door. It slid open despite the console never updating or changing.

Tom's right, the door notice doesn't change, and, in my game at least, even when you walk right up to the middle of the door it's a little slow to respond... just put your nose on it, so to speak, and hang out for a second or two, and it should open, it you've passed the index mark...