Measure like you are in the tub, over the tape, under the paper.

Don't worry about the paper towel. Once the snake in laying on it the heat will transfer to it. Right now there is an air gap so the paper towel is not hearing up. If you want to test it put something heavy in it and measure under it after it sat there a bit.

I can't tell you how long it will take to reach your set temps but it sounds like it has.

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BTW, heat tape is not meant to heat the air and will do almost nothing to raise ambient temps.