This is my incubator it holds 20 ((10) 7 quarts + (10) 15 quarts) to 30 clutches ((30) 7 quarts)
The thermostat probe is place in the middle tub, and I have a thermometer in the top middle tub as well as the bottom middle tub the variance is less than 2 degrees from top to bottom.
I do have a fan on the top circulating the air.
The temps IN the tubs vary from 88 at the bottom to 90 at the top.
You need to place both T-stat prob and thermometer prob in the egg box preferably taking the temp in more than one egg box.