HVAC pros

It has been extremely hot / dry locally here .. We do get into the 100's in the summer for a few days often but, this year we seem to be stuck in upper 90's to 100 day after day

Talking to a person last night , he said he only sets his t-stat to 8 degrees below expected high of the day .. i said so, like 88 ?

Claims he was told to never expect it to cool beyond that ? I guess this is news to me ?

I was always said to expect at least 20-25 degrees from outside temps ?