Since T-Max is so sensitive to temp variations, use whichever temp is easiest to maintain for the entire development time.
In the Book of Pyro, Gordon Hutchings reports good results with processing at 80 degrees with PMK, as long as it is not for minus development.
Normal development for T-Max at 70 degrees he recommends 12 minutes.
Normal development for T-Max at 80 degrees he recommends 9 minutes.
These are starting points, of course.
And he says for each degree of increased developer temp, decrease development time by 4%.
He also suggests letting the mixed developer stand for about an hour before using in order to increase stain when developing T-Max 100.
I highly recommend the Book of Pyro as a very useful reference for all things PMK.