Planning a house build for this spring in zone 7 (8960HDD). 2,000 square feet with a 1093sf unfinished basement. 

I am attempting to get a reasonable sense for heat loss in our house design to aid in mechanical and component selections (heat system, windows). Here is what I have found:

Free Excel Sheet from Marc Rosenbaum
I found a free Excel spreadsheet with the typical inputs that is meant to give you a quick and basic calculation. I’ve attached a screenshot. I am able to pull the volume and surface specs directly from my SketchUp model. 

Any sense for how accurate this tool might be? Is it possible to have a reasonably accurate calculation with such basic inputs?

I like this tool as it allows for easy tweaking to your building components and factors. There are some inputs that drive the results in a huge way (like airtightness of course) but it seems that things like wall insulation have a much, much less dramatic effect. 

REScheck
I also ran all of our specs through REScheck (front page screenshot attached) since we’ll be required to do so by code anyway. I was surprised that REScheck did not provide me with a heat loss calculation but rather a UA (building load coefficient). It appears that I can calculate a heat loss using UA (I think by multiplying it by design temp difference). 

How do I get a heat loss in BTU/hr from UA?

Is REScheck (UA to heat loss conversion) an accurate tool for heat loss calculation?

REScheck shows a UA much lower (259 vs. 414 in Excel sheet), I assume that makes more sense and would be more accurate, right? More detail input, more accurate outcome?

Decisions to Make
There are two main drivers to this: 
1. I need to make a window selection and would like to understand how different window specs impact overall performance and heat loss to aid in my decision. See recent post: https://www.greenbuildingadvisor.com/question/help-me-choose-my-high-performance-windows-please

2. I would like to confirm whether or not we can heat with air-source mini-splits and if it will actually be comfortable. 

Does anyone in zone 7 have experience heating a home with air-source heat pumps? We just had a period that remained between 0 and -25 for roughly 7-10 days and these are the periods I am most concerned with.

Any other insight on heat loss calculations, window selections and use of air-source heat pumps in zone 7 would be greatly appreciated!