A typical tight 1950s 2x4 rancher with clear glass storms over wood-sashed double-hungs will usually run between 15-20 BTU/hr per square foot of conditioned space @ 0F, depending on whether it has an un-insulated vs. insulated full basement (or sealed crawlspace), and the total number of occupants, plug-loads etc. At +15F (coastal NJ) that would be more like 12-16 BTU/hr per square foot. So it's likely your true load is in the 25K range, but if you have some old gas bills that can be verified with a
fuel-use based heat load calculation , which is simpler than a Manual-J.
Without the load numbers and the radiation sizing there is no way to suggest temperature points on the curve.
The whole point of a modulating boiler with outdoor reset is to have long steady burns, letting it modulate the firing rate up /down with the outdoor temperature.
The English language versions of
the manual and control are set in Fahrenheit degrees. << The outdoor reset discussion starts on p56 of that manual.
Since the recessed cast iron rads (right, not fin-tube convectors?) have fairly linear response even at low water temps as well as substantial thermal mass it would be fine to set the minimum output temperature to 100F or so- there's not much efficiency to be gained by going lower. If that keeps up and doesn't short-cycle the boiler great- set it for 100F at all temps and just leave it there. Odds are 100F isn't enough though, and you'll have to figure out a couple of points on the curve.
The displayed parameters you reported are, according to the discussion starting on p54 mean:
A 118 < --- (= the current space heating output temperature setpoint if under supply output temperature control)
B 93 < --- (= the current space heating return setpoint when under input temperature control)
C 102 < --- (= the programmed domestic hot water output temperature)
D 74 <--- (= the sensed incoming domestic hot water temperature)
E 0 <--- (= the sensed domestic hot water flow rate, which being zero, explains why the incoming sensed temperature is so high.)
F 22 <--- (= the sensed current outdoor air temperature)
G --- <--- (= outdoor reset not currently in use set it to either 3 or 6, for cast-iron radiation- try 3 first)
H 0 <--- (= boost interval time. Setting it to zero turns that function off, which is fine.)
I 12.6 <--- (= heating system pressure, in pounds per square inch- 12.6psi is just fine.)
Try programming the low temp at your your 99% outside design temp (guess, based on the nearest listed city), and set the highest temperature to 60F (a guesstimate, based on where you set your thermostats it'll probably do just fine during the colder seasons. If it keeps up, drop the programmed lowest temperature by 10F, see it still keeps up. Repeat until it doesn't keep up with the load over night, then raise the low temp 5F, and bump it up or down a degree at a time until it keeps up.
In the spring as temperatures rise keep an eye out for short-cycling behavior. Burn times will be shorter when the load is low, and is guaranteed to cycle on/off above some outdoor temperature. As long as burns are at least 3-4 minutes long, with no more than a handful of burns per hour it'll be just fine. But 1-minute burns and 10 burns or more per hour are an efficiency & wear & tear disaster. The thermal mass of the radiators will probably keep that from happening, but be aware of how it's behaving when it's say, 45F or higher outside.
The 5F setback function on the Nest will screw this all up, and won't really save you much, if anything. When you have it dialed-in without the reset function, you can try enabling that function on the Nest, and enabling the Boost function, set to 15 minutes or something.