It'a teeter-totter balancing act... Basically you assume the house is going to be a steady temp, then you punch enough holes in the incubator so that the light bulb raises the temp a certain number of degrees via a mixture of just the right amount of outside air...
So for example, if your house is 70° you punch holes in the foam until the light bulb raises it up to 100° aka +30° the problem is it's fully dependent upon the house staying the same temp... If you house temp drops to 65° then the +30° results in an incubator at 95°, and the same is true in reverse, if the house temp climbs to 75° the +30° means the incubator is now at 105°...
IMO it's simply not worth the trouble or risk to deal with it, as you said for $8 you have no balancing act and it simply works effortlessly and you will have better overall hatching results in the long run...
