I had a severe deformity in a dead hatchling recently, and my fluctuations were about 4 degrees. So i'm going with anything over 4 degrees is probably pushing your luck.
I set mine for 2 deg offset just to keep if from going on and off nonstop, I had a few minor tail kinks thats is about it. maybe I should kick it up to 1 just to see if everything turns out perfect . I put a probe in the perlite with the eggs a few times just to double check, and it did not move from 81.3 for the times i checked on it. a cordless usb temp data logger would probably be more accurate. I will have to get one, one of these days. I never thought I would have needed one untill I found out they existed.