The problem is the heat tape is just not enough wattage to make it up to temp. The power matching on the Herpstat only affects the power when the temps get within a degree of your set temperature. The easy way to see the problem is to unplug the heat tape from the thermostat and plug it right into the wall and monitor it. Just leave the Herpstat's probe in place so you can monitor the temperature on it and you'll see the heat tape not make it to temp. 70 degrees can be tough for heat tape to overcome. For instance I have 1 foot cubes built out of 3/4" wood for test setups with 1 foot of 11" heat tape. With my room temp around 70 these cubes (closed off) cannot make it to 88+ degrees. For your situation you will probably need to up the room temp, add additional heat tape, or swap to heat tape that has more wattage per foot.

Dion Brewington
Owner, Spyder Robotics