The Proportional option on that t-stat should give you a more consistent temperature inside of your incubator compared to the on/off style. Proportional is designed to use as little energy as possible to maintain the temperature while the on/off style will stay on until the desired temperature is reached then switch to off. The on/off style has a tendency to over shoot the desired temperature which is why you are getting the variance from 88 to 91. I always use proportional on all of my t-stats.
I place my t-stat probe in the top of my incubator, outside of the egg boxes. Once the whole incubator heats up the egg boxes along with the rest of the area inside the incubator should be fairly close to the desired temperature. It is a good idea to run your incubator ahead of time to work out any kinks you may come across, that way you aren't messing with it when the eggs are ready.
Hope that helps you out as much as possible!
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