Kind of following up on an earlier thread with a slightly different question.

I could not find a contractor willing to do a load calc for me, nor do I have the patience to screen them, so I did my own using HVAC-Calc, even accounting for kneewalls, insulating air pockets, kitchen cabinets, and future work. I figured nobody knows my house as well as I do, and with the cost of energy, I don't want my system supersized.

The final number surprised me: just under 60K BTU/H for an uninsulated 90 year old, 2000 square foot stick-and-shingle home in the Seattle area. It's surprising because the 22 year old locomotive in the basement says 105K @ a measured 78% efficiency (~80K). On the other hand, at nearly 30 BTU/sq. ft., it's almost 3X the published target for well-built new houses here.

Can I trust the software to be right, assuming my inputs are correct? I'm considering a new 75K unit @ 92%, giving me a 15% margin before insulating.