Heat loss

better than guessing. The more data you can enter into a program the more accurate the results.

But a number of assumptions need to be made regardless, unless you do a blower door to nail down infiltration, maybe an infrared scan to check insulation quality.

For existing structure, the quickest and most accurate heat load is from fuel use. Older houses have too many unknows for any Man J type of calculation to be accurate. In case of fuel use about the only error is boiler efficiency, DHW loads and passive solar gains.

The math is pretty straight forward:

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

If you want even quicker, in zone 5,6, winter fuel use in Therms X 42 is about your heat loss. So if you burn 1000Therms of gas, your heat loss is around 42000BTU.