The best rule of thumb for heating an incubator is to only use enough heat tape that when on full power will only get the incubator upto in around 100F. That way if you have a thermostate malfunction and the incubator sticks on high you won't kill your eggs right away.
It really sucks waking up in the morning and checking on the incubator only to see it sitting at 125F.
Also put a small computer fan in there to help maintain an even temp.
Corey
Yep - I'm heating a huge marine cooler with only 1 ft of 11" flexwatt and so far it has been the perfect amount of heat...