I have 3 ranco's in my incubator. One for normal operation at 89 degrees, one that kicks on if the temp drops below 88 degrees, and a third that turns the whole system down if temps reach 91 degrees. I keep them all at 1 degree differential. The temp readings on them all were pretty much dead on with each other and when I stuck a calibrated thermal couple in it came up the same. From this experiment I would say the rancos are pretty accurate. If you can safely attach it to the top of the glass in the bottom of the tank over the heat mat, I would not be afraid to believe the setting on the ranco. Make sure it is firmly pressed against the glass though.

One problem I do have with the ranco is the relays recommended wiring method. If this relay fails it will more than likely fail in its normal state which is closed! This will set the heat element full on unregulated. It may be better to use the Normally open contact and set the ranco to cooling mode. This way when the temp drops below a certain point the ranco will pick up and start the heating element. When your given temp is exceded the relay will drop out and stop powering the heating element.