Heat cable?

I want to start converting some of my snakes into Sterlite tubs, and eventually to some sort of rack system. What length heat cable should I get to heat 5-6 large Sterlite tubs (40+ qt), and do you just coil it under the tubs like a heat pad?

I use this stuff under my tubs http://www.reptilebasics.com/4-Flexwatt-Heat-Tape-p-16248.html

I use a large rack from lowes and put my tubs on there. Get heat tape as long as the rack shelves.

Although I must be honest: as I have been getting more snakes I bought a room heater and just keep the room at 80 degrees for the corns and put heat tape under the Ball Pythons.