I can find/closely estimate the heat loss easily. What is much harder to find is the heat gain/transfer into the room from 1920s cast iron radiators at 135°F flow and the balance of the system flow temperatures at those 45°F lower flow temps than originally designed.
Then, because the answer is almost always going to be "yeah, it's going to be really close...", I felt well-advised to prove it via experimentation rather than commit to changing the heating plant to a system that could not provide 150°F flow temperature.
The question is not about the general hydronic heating formula [nor manual J heat loss estimations], but rather "what will the delta T of the rad in this particular room, in this piping network [it's a converted gravity feed system, now being a pumped], using 69°F room temp and 135°F leaving water temp from the heat source?"
This is going to be a complex problem because of the shape of the radiator and you'll need to calculate radiative and convective components of heat transfer i.e. you'll need finite element analysis to do this. If you simplify it to a simple shape like a rod or slab you can get somewhere in a calculation, but this is only going to give an instantaneous measure because of heat transfer to the rest of the universe.
Alternately, to get a realistic measure, you'll need to set your boundary conditions about what the heat flow out of the room will be, which is a bit simpler to setup with U-values, area, delta-T, and heat capacity of materials. You'll also need to do this to every other room in the building simultaneously. This is a Manual J, or heat balance method or the radiant time series method load calculation that will balance out with the amount of heat leaving your radiator without knowing its specific shape.
For the, for lack of a better word, standard radiators there is a formula with a dT^4. But I totally agree, this isnt all that straighforward, given that for the dozons of installers, experts and home owners I have spoken, Ive heard dozens+1 methods for estimating. Estimating heat loss from a given building and estimating power output of a given installation of radiators, very few people seem to be able to calculate that.
If your heater can go low (mine bottoms out at 50 unfortunately), by far the easiest is to just test.
Then, because the answer is almost always going to be "yeah, it's going to be really close...", I felt well-advised to prove it via experimentation rather than commit to changing the heating plant to a system that could not provide 150°F flow temperature.