temp problems

[V.hb]

Having some trouble keeping my incubator at 86 degrees.

I'am using Jeff Favelles type of incubator, as posted before in this forum. The water heater iam using has temperature settings of 85-90. So when I set it for 85, i get egg box temps of 84.. with 90, i get 87.. Any ideas of how to keep the temps more accurate?? Any help would be great, thanks!