When I was little, our incubator was a small rectangular trashcan and my old bedside lamp. Mom kept a thermometer on top of the eggs and adjusted the lamp until all was perfect. I had what must have been my first Blonde Moment: I turned off the lamp. I mean, my parents were always yelling about the electric bill and here was this light on... When Mom and Dad got back home I got an earfull. I went crying into my room, convinced I had killed all the unborn chicks. After a parental conference, one of them came in and said things were most likely all right. (It's been years and who said exactly what isn't that important.) "The old hen gets up to grab a bite to eat, some water to drink, and poops while off the nest. Hatching eggs don't stay 100 degrees exactly for those 21 days."
If you don't have an auto-turner, the temp drops any time you roll the eggs. I keep a notebook with the times I rotate and the temperature updates. It's been extra cold here, so the whole house is colder than normal. I layered flannel tablecloths under the incubator and around the side away from me. Even if a Mama hen was sitting on these eggs, I would bet they wouldn't be exactly 100 degrees with it 26 degrees outside. "Best" and "Optimal" are just that. We do our best since we are not sitting there saying "That one under my right leg seems cold... I need to move it to the center and warm it up..."
Best of luck to you!