Tricking the packaged software

I'm using SlantFin's free download, "Heat Loss Express," to try to analyze my project.

I'm wondering how to handle the large irregular common space on the main floor. See it in the attached pic.

Inputs are 1. ceiling height, 2. room length, 3. room width, 4. window area, 5. exposed wall length, plus the usual assortments of "factors" for thermal transmittance of each of the barriers, roof, wall, glass, yadda, yadda, yadda.

Thinking the program does a calc to get the ceiling area by multiplying L x W, I fudged the program by giving a L and W that when multiplied, yield the area of the whole ceiling for the irregular space. Other than that, I plugged in everything else as true.

My dilemma is this: I gave all my data to a heating pro, who allegedly ran calcs with HIS package, and came up with a 27,389 Btu/h loss for the space, while SlantFin's program yields me 35,196.

The other thing that has me worried about what my "pro" did, is a basement bathroom, no window, only exposed wall is ICF block foundation, fully buried, he says loses 1481, versus 1597 for a bathroom above it that has two windows, a woodframed exposed wall, and a cold attic above.

For those two baths, my calcs came to 662 and 1598, respectively.

I'm worried about what my "pro" is coming up with, and he's the only pro I've got.

What to do?