IMO 77 cool side is fine. Usually I don't worry unless that drops to below 75. I find that even with all my temps dialed into the usual 90-80 split, my boas choose the cool side much more than the warm. This used to drive me insane years ago, until I spoke with some veteran boa breeders and they observed the same behavior.

I even got to the point where I had friends come over and take temp measurements with their equipment to make sure that my temp guns weren't off.