Mea Culpa--sorry for my inadvertent hijacking, so let me repeat my post:
The only way I could get a heat calc done was to do it myself. My loss came to 60,000 BTU. According to the author (of the software), one should allow a safety factor of 20 or 25%.
One furnace that I am considering, a Rheem Mod 75,000 unit (upflow), has a high stage output of 72000 which would be exactly 20% oversize at design. My understanding of the furnace is that it would probably not operate at full capacity except maybe for recovery from setback (11 degrees). While this hasn't been the coldest winter in memory, we had a cold spell which was 4 degrees below design, and the Mod could handle a difference of 10+ degrees below design.
I had originally considered two 80,000 units but because excess sizing, and of blower specs, I'm leaning toward the Mod.
Since we normally keep our occupied temperaure at 66 degrees, it would be necessary to lower the setting in the event of temperatures in excess of 10 degrees below design. In looking at the discussion here, I haven't seen any reference to a safety factor and am wondering what emphasis you put on this, or to put it another way: how much safety is reasonable?