On the paper towel, should I measure the tub itself?
Always measure the tub itself. These guys like to move whatever substrate you're using, so you want to get a reading on the hottest surface they can come into contact with (the floor).
Hot spot of 85 ok on the paper towel? I don't think they are smart enough to move the paper towel.
1) They are smart enough. 2) When they lay on the paper towel, it will be warmer than the paper towel just resting on the tub. 3) Even if neither of these were true (they are), 85 is still fine.