Get at least five thermometers and place them side by side for calibration. Leave them to stabilize for an hour or more. Mark them so that they are all in reference to one (I usually choose the middle one)., and place one in each corner of the incubator and one directly under the fan, all on the floor.
Leave for an hour, and gather and adjust the readings. If all corrected temperatures are within a degree, that's good.
Now, if you are using a turner, remove the first turner rail next to the motor if possible (to keep the rails from turning), place the turner in the incubator, plug it in (the turner motor creates a lot of heat) and place your thermometers around or tape/fasten them onto the rails so that they do not interfere with operation if you can't remove the first rail. Again, wait for the turner and thermometers to come up to temperature, at least an hour. And again, gather and adjust the readings.
If there are no hot spots and no cold spots, you are in great shape. Now all you need to do is set the temperature.
More likely, though, you will find the corners cold, next to the turner hot, and random hot or cool spots. You can set the eggs around these hot and cold spots, or rotate them during incubation so that slower developing eggs are moved to warmer spots.
It takes very little air movement to break up the temperature striations, which is the first goal of a circulation fan. Just like a ceiling fan, the lowest setting helps even out room temps. With a 12v computer fan, I use a 6v or 9v power supply so that they turn much more slowly. I'm not using them for cooling or exhaust purposes. If the fan spins too fast, it may also be harder to manage humidity.
Thank you so much