Sounds like you're on the right path with your heat problem, just try to get it down and keep it down, fluctuating temps, especially that much, is an invitation for mutation, you don't want hermaphrodites!

I'd use about a gallon per plant for a flush, just don't pour it all in at once. I used to put mine in the tub and do it that way, no mess to clean up..

Good luck

Peas out