Hope your chosen user name isn't short for "Brother's Broke", but could be if your using propane, it's hit a high price here in Mid-Tenn.
Anyhow, the best way I figure out your answer is running a load calculation, that is how much heat is required at 0 degrees outside and 70 inside, then you could divide that number to get the amount of btuH's needed for the heat rise, but it's usually a curve on a graph, that is it takes more heat energy at 0 to get started, then of course less as you get warmer.
But really, a load calculation can give you just what you need, btuh's to be 70 inside with 0 outside.
Try here: http://www.getfreesofts.com/soft/427..._Software.html
and see if it will work for you, if not, do a search for "free load calculations"