In Ontario, we use a sanctioned set of calculations that takes into account the exposed area of each wall, roof and floor, factors in orientation of each surface and makes a parallel set of calculations for the windows and door openings.  In Ontario, the institute managing this process is HRAI and installers/designers should carry an HRAI card showing that they have passed the (non-trivial) exams.  I'm quite sure the HRAI process is based on similar ASHRAE-prescribed processes and other jurisdictions may refer directly back to ASHRAE or to some other local authority which in turn bases its process on ASHRAE, much as HRAI does.
In your case, there are two sets of calculations required.  One is, as you point out, the HL (and HG for summer) calculations (HL = heat loss; HG = heat gain).  The second set is to calculate the layout and BTU-delivery capability of your radiant system and identify those areas (lots of glass, typically) which will need supplementary heat, such as higher-temperature radiators like the models of Runtal.
I'm authorized only to do the first set of calculations in Ontario.  There are usually independent guys who work free-lance for heating contractors and who might be able to help you out.

Just checked where 49 th parallel was, thats a cold area, not sure if this will help you ...but in Pennsylvania the equation is sq ft times ceiling height times either 5,6,7,8 thats the estimated insulation in a house, example...old house with little insulation would be higher #=8 new house with good insulation =5 or 6. so if your house is 1500 sq ft times 8 foot ceilings with good insul (times 6) 1500 x 8 x 6 = 72000 btu's ! But again thats down here! I hope this helps in some way

You can contact the service provider.......