@electrotechs I’ve calibrated the soldering irons today.
The set temperature on them both was 400ºC.
The left iron was reading 350ºC, it now reaches 400ºC correctly.
The right iron was reading 310ºC, it now reaches about 390ºC after two adjustments.
I would suggest that the right hand one is probably good enough now.
I will recalibrate again in the next month to see if they’re drifting.