If your boiler is running at 60°C, ideally, the return wants to be 60*0.7=42°C - If the return temperature is higher, then I’d suggest looking at the balance of all radiators and possibly reducing the pump speed if you can (I have my pump on the lowest setting).
Condensing only really kicks in at 54°C, so the lower the return temperature, the better. At 55°C, you’re probably only getting 86%, so plenty of scope to improve on that.
I have a heat meter (an L&G T230) plumbed in to the heating system. This gives a pretty good idea of how much heat is being put out by the radiators. If you know the flow & return temperatures along with the flow rate, it is a trivial calculation to work out how much energy has been consumed. From my notes:
The specific heat of water is 4190 J/(kg⋅°C). It means that it takes 4190 Joules to heat 1kg of water by 1°C. Heat required to raise the temperature, Qt: Qt = cm*ΔT where: Qt is the specific heat capacity; cm is the mass; ΔT is the temperature differential Or: kWh = cm*ΔT*c c is specific heat of water divided by time: 4190/3600 = 1.163889
Looking at tables for my radiators, I know what each one is rated at based on a flow temperature of 70°C (often quoted as ΔT50). Using correction tables (e.g. https://www.clyderadiators.co.uk/delta-t-conversion ), one can work out how much heat will be radiated at a particular flow temperature. I have a total of 12.5kW of radiators, so at 40°C (ΔT20), my limit is 3.75kW - These figures are based on a room temperature of 20°C.
When a TRV (old skool 0-5 numbers) kicks in, it stops water flowing through the radiator. Effectively, taking that radiator out of the equation. As a result, the system is not able to dissipate as much heat.
Below, a plot showing flow/return temperatures and power dissipated by the radiators.
