Yes ... it's very much a matter of trial and error ... it took me a while to find just the optimum amount of heat but you are right ... a Tempilstick would give you some confidence that the temperature was about right for sublimation:
We know that sublimation of OA starts at around 150 degrees C (not that hot actually !) and if you could establish what this was like in terms of the look of the copper pan with the aid of the Tempilstick it would take some of the guesswork out of it.
I have seen people heating the Gasvap until the copper almost glows red and it really does not need anywhere near this level of heating to get the OA to sublimate ...
https://www.1stopweldingshop.com/Catalogue/Welding-Supplies/Arc-Welding-Accessories/Temple-Sticks