Have any of you done a heat loss calc. on a building, then sized the baseboard from that?
Yes....on a room by room basis stat zone controlled. It is very difficult to determine heat loading by single volume extrapolation in residential design unless the structure was one room with same ceiling height without windows.
In the beginning days of CA Title 24, heat loss calculations included using the crack method for leakage loss and insolation rates required for heat gain through glass to floor mass where there were no fenestration ratios considered.
There were no Climate Zone environmental knowns established in the early 60's either. Now the architectural guidelines are based on the Federal mapping along with State adopted energy conservation Climate Zone regulation with options for calculation by design mostly.
The days of using square footage to volume for residential energy calculation in new construction is disappearing for heating and cooling. I can predict more complex rules will be coming to your neighborhood eventually if the Feds have their way. rbj