I have a walk in incubator and get flucuation from 88 to 91 at times and never had a problem. This is one of those topics that can easily become an overthinking issue.these animals have been reproducing in the wild for along time and im sure temps dont stay at a constant 89. I know of other breeders i deal with that are actually incubating at lower temps in the 87 degree range and finding the babies stay in the egg longer and come out bigger. Up until about 5 years ago i used a freezer incubator and had some flucuation and never noticed issues. As long as your temps are not spiking and staying at the extreme of the flucuation your fine. Also it is important to know exactlly what the temp is in the tub. I have a dummy tub mixed with vermiculite and i put the stat probe in their so i know exactlly what my eggs are getting. Also its important your fan is running constantlly instead of having it kick on when the heat kicks on. I also have 6 filled 5 gallon buckets with tops to help with keeping constant temps. Now granted its a walk in so i need the buckets but you just scale that down for a smaller bator.