I think that no matter how much flex watt you choose to go with, ambient temps will be a real pain to achieve...
Do you have your flex watt? If not, you could order it from Rich at RBI and ask him what he might suggest.
Also, have you considered a RHP? They don't dry the air and they totally bump up ambient temps. I live in a cold place and have RBI cages that I also equipped with RHP's since the flex watt was just not doing it for the ambients.