Ambient room temp required to allow flexwatt in a rack system to be effective?

Using flexwatt heat alone for a rack system with 6" deep plastic tubs, about what ambient temp would I need to keep the room at to allow the tape to reach that 88-92 degree hot end and allow the cool end to be 78-80?

I keep mine around 75-77 using 4 inch flexwatt.... It work good for me with that size tub...