Its possible to make a stab at estimating the number of good callous hours from the Weather Underground plots. Here are my current plots from my backyard for the most recent week for example:
The sun intensity is the lowest line, the yellow one. You can see that from about midnight to 5AM today (the 14th) I had near perfect callous temps, and it was also nearly perfect for about 12 hours after I did the grafts on Thursday. I also had around 8 hours of bad over-heating on Friday. The goal is to maximize the hours of optimal ~70F on the grafts while minimizing the overheating hours. The hard question is which is most important - grafting at colder temps will minimize the overheating but will also give many fewer good hours. The night-time hours in particular are really good on warmer days, I have accumulated many good hours in the last two days at night.
A long time ago I found a picture of peach callous rates at different temperatures, but I can’t find that graph now. It would be good to have that data; my recollection was there was no callousing below about 50F and above about 80F; I don’t remember how quickly it tapered off though.
One other factor I am going to try to measure his how much the foil helps. I was surprised at how consistent the temps I measured were and I didn’t notice a lot of different of foil vs not. The exposed trunk would heat up the grafts even if they were under foil. It suggests that painting the whole trunk white or covering it all with foil could be a good idea in weather that is too sunny. Or, just block the sun with a barrier of some sort.