The general rule of thumb is the heat tape should cover about 30% of the tub, but that can't always be exact. I use 6" in my 28 qt. rack and 12" in my 41 qt. rack. For a 34 qt. rack, I would probably go with 6" just to make sure there is enough gradient in the tub. What are you putting in the tub? How large is it? If it is an adult Ball Python, I would get a 41 qt. tub and use 12"...

By the way, THG is my personal preference for heat tape, because it's much easier to wire up and it's stronger than Flexwatt. With Flexwatt you have to solder the wires on to it and with THG you don't...