TBH it's probably easier to measure the temperature of the radiator
with a proper thermometer then use that to calibrate the IR meter!

Completely disagree with you, I'm afraid. IR thermometers are far more
accurate, reliable and, most importantly, rapid than any conventional one,
provided you understand how they work and how varying emmissivity effects
readings.

You only have to use ones that measure human temperature to realise that an
IR gives clearly an accurate reading, whilst a non-IR one invariably
underreads. I have built an IR thermometer from scratch 10 years ago at
university and understand how they work.

To get accurate results on metal pipes, it is advisable to reduce the
reflectivity of the surface, and just as importantly provide a fixed
emissivity for comparison. This is easiest done by applying some black tape
and aiming at that.

Conventional thermometers are a faff and complete waste of time.

Christian.