The easiest: do some tests, monitoring the temperature of the room. Get the heaters on full blast, then check the temperature, switch off the heaters, and see how much the room continues to heat up. If you switch off the heaters at say 20°C, and it heats up to say 22.3°C, then to get to 25°C you should switch off the heater when you reach 22.7°C. The expected 2.3°C extra heating should get you very close to your desired temperature.
After that you get to the much different problem of keeping the temperature at that level, or at least as close as possible to it. The inherent delay in your heating system is one of the difficulties there: it takes a while between switching on the heater and the hot water reaching your radiators, just as it takes time for the heating to stop after switching it off. That's where PID would come in play.
The location of your sensor is of course critical, as different parts of the room may heat at different rates (fans can help here - both to distribute the heat, and to blow air over your hot radiators enhancing the heating effect - making your room heat up faster as well).