Ray is right, knowing the minimum you want it to drop to, as well as the lowest it will get to outside, and how good your greenhouse is at keeping in the heat are all factors.
Great calcuator Ray, I've not spotted that one on your site before. I'm doing more of a guess and hope method
For me I only want to stop it freezing in there over the winter, so don't need as much as I would if trying to keep it up to temperature for the orchids I keep in the house.
I save on heating by insulating the greenhouse with bubble wrap every winter. It makes a difference but I have to empty the whole place (benches and all) twice a year to put it up and take it down. Just starting to think it's time to take it down again now.
I know a previous member of Orchid Board (Kavanaru) did a lot of work with digging down the foundations of his greenhouse deep into the ground, because using insulating materials lower down was going to help save on heating. He was trying to heat for temperate plants over the winter though. Unfortunately I can't track down his posts on it just now.