A true 40BTU/ft heat load would be the heat load of an uninsulated room with single pane glass in a MA location. A floor emitting 40BTU/ft would be an uncomfortably hot ~90F (usually tile) surface that feels a bit like hot asphalt on a July afternoon to bare feet. I'm going to assume that you're just trying to match the room-to-room balance, and don't have an actual heat load of 40BTU/ft.
The air space between the floor and most of the c.i. tub will have an approximate R-value of R2-R4. If there's nothing but flat plank, ship-lap or t & g plank flooring figure on an R-value of the planking of about R1.2. If 3/4" plywood, R0.9. Getting 40BTU/ft through ~R3-R5 is going to take water hotter than 160F. But at least the tub won't get too hot, eh?
A better approach is to either install a convecting panel radiator in addition to any floor radiant to approximately match the output curves of the fin-tube, or to install radiant ceiling in addition to any floor radiant.
Hopefully you didn't succumb to anxiety and install anything bigger than the Solo-60, unless you did a careful heat load calculation that prove you're in one of the ~2% of MA homes (that's a WAG, not a hard statistic) that might actually need something bigger? Oversizing it the most common error with mod-cons for both rookies & seasoned HVAC installers alike, but to get the most comfort & efficiency out of it requires down sizing to the smallest unit that will actually serve the load, so that it can spend most of it's time modulating rather than cycling. The -60 is already more than enough boiler for the vast majority of homes in MA. The minimum fire output of the -110 is more than half the true 99% design condition heat load of the average home in MA, and well above the average condition heat load.