What I've found is that as long as the room temp is at least 10 degrees below the desired incubation temp, the setup is rock solid.
If the room temp for example, gets to 80 or above, and the desired temp is 88.9, the unit will spike up to 90 or over. Through trial and error, I can attribute this to heat that is actually put off by the fan motor.
While it is not perfect, in reality, it has never affected hatching. I, however, resign myself to working in and "unperfect" environment, much as nature.
It can be resolved by killing the fan when you "have the a/c off" on hot days, but I rarely do unless the incubator hit's 93.
( let the boo's begin... lol. i'm a bit more careful on chondro eggs than bp eggs, but my hatch rate is consistant in both species. )
So to the point, that one foot is more than fine for that cooler setup.
I have actually lucked out this year on spacing of clutches or I would have had to build another one.
Thankfully, that is still keeping pace.
-d