The original baseboard may have been overkill for the actual load. (usually is, by 2x). Measuring the baseboard is a downright
lousy way of estimating the design heat load, even worse than stupid-on-a-stick "BTUs per square foot" rules of thumb. Your real heat load for that room/area is almost certainly under 3000 BTU/hr, and could easily be half that. (Reality is probably is about half, using experience as a guide.)
If you want to go there on a dumb rule of thumb, (and I DON'T recommend it) a not too drafty 2x4 framed insulated house in Littleton with clear glass double panes would typically come in at about 14-16 BTU/hr per square foot of conditioned space, a 2x6 framed house with low-E double panes 10-12 BTU/hr per square foot of conditioned space.
If you have a heating history on your house you can run a
fuel-use based load calculation, and if the temperature balance room to room has been reasonable, use the proportional baseboard lengths to come up with the proportional heat load for the area you're looking to heat.
If the joist bays under the floor are open and accessible, suspended tube radiant with your higher temp water is cheap and DIY-able, and a lot nicer. (An engineer should be able to do the napkin math on that without losing brain cells.
)