Incubator finished but need to know temperature variance tolerance

[Engteacher]

Woot! My son and I managed the wiring and did a 5 hour test run. We have to crank the thermostat up a bit, because it switches off at 97.5 F and then kicks back on at 90F. How much of a temperature swing can the eggs tolerate? I did add water in baby jars to work as a heat well. I did find this thread to modify a water heater thermostat.

https://www.backyardchickens.com/web/viewblog.php?id=26580-thermostat-modification

Do you have any other tricks for stabilizing the temperature in a homemade Gopher-Bator?