Youll be sorry with 65watts.
My P13 (88 watt) in my T12's were fine when the house was 68+ but at night when the house goes to 62-65 the cool side dropped to 73. A 65 watt wouldn't do anything in a T12 unless you heat the snake room for ambient to 70. He sold 88 watts for T10's last week.. Better call him back. For a extra $20 get the 88watt. You wont be sorry.
I have Central Air in the summer and heat goes down at night during the winter so he had me trade him back for the larger ones.