11 inch would work fine for cb-70, or two rows of 3-4 inch tape even though it would be more wires, you cant really go to big on the tape as long as your only heating 1/3 of the tub. Example 3 inch would do the trick, so would 11 inch but the 11 inch would just seems more evenly spread heat, and require less energy to heat to your specs. also if your ambient temps are 70's which mine are aswell, thenm using the 11 inch will be better (wont need a space heater)