Just what is heat loss calculation used for?
Is it just for sizing the boiler?
If so can I use a program that is for calculating forced air and baseboard heating.
Isn't heat loss calc, heat loss calc no matter what it is going to be used for.
I see that in one of the sticky's that it is mentioned about doing the heat loss calc. yourself. But doesn't mention what to use.

load calcs determine everything: what water temperatures you need, what your zoning should be, what your flow rate requirements are, what size heat source you need, perhaps tubing spacing or installation method requirements, and with some degree day analysis it can steer your class of heat source decision as well.

you can use anything, but most load calculation software overestimate loads significantly. and it's definitely a question of GIGO: garbage in, garbage out. If you don't know what you're doing, it's unlikely that your load calculation will mean much.

Thanks
I had heard that with an older house like I do, 71years old.
A value of 20BTU / square ft. is a safe bet.
Does this sound like a rough estimate?

it depends. in old houses it can vary from 20 to 40 BTUs/sq ft, typically.

The walls up and down are insulated.
75% of the windows are double glazed.
Attic is story 1/2 type there is batts on the pony walls and blown in insulation on the floors but the slopes have minimal insulation.