This is a pretty nice tool. Not as detailed as the Manual J calc... for instance it does not let you specify which direction windows are facing. But for me it was just fine since I have about equal glass facing north and south. I think it captures the envelope of a structure pretty well. And the price is right. As usual, your mileage, and BTUH, may vary.
Oh, it isn't obvious from the input form but the calculator assumes an interior temp of 70 degrees F.