Hey GBF, it all sounds good to me, you been doing your homework.
Temps are perfect at 75-80. I wouldn't worry too much about the heat when you go into flowering, you've got a good fan (although i can't be sure without doing a bit of research but it sounds about right) just remember to take into account the loss of power from your ducting and sucking air from the 2 cooltubes, are you using a Y connector for that? so you should have a fan that's rated higher than the room size and the cool tubes should help a lot too. When you switch to 12/12 depends on your height restrictions (how big can you physically let them grow to?) If when you take into account the height of the pots and room for your lights and distance from your lights to the tops of your plants (i'd guess you could get them to within 8" with the cooltubes) you've got say 3ft of height left to play with i'd start flowering when they reach about 12" high, (probably about 4 weeks old) they usually double or triple from then until harvest. (but it depends on the strain, an indica will be closer to doubling in height while a sativa will be closer to trebling.) You just have to practise till you get to know the strain and how much it will stretch so that you can grow them as large as your area allows. Hope that helps.

Good luck!