its not just temperature that you take into account, its also humidity, barometric pressure, intake air temperature etc..... so determining the absolute temperature that they zero calibrate at isnt as simple as it seems.
If your car starts to heatsoak on the dyno then the dyno needs better cooling, ideally a dyno run should match as closely as possible to a road run in both intake temps and pull time, but most dont. We spenat ages testing different fans and even though we now have an OK setup it still isnt ideal, so I have now invested in 2 more fans that are capable of this:
I predict a few number plates going on a journey