heating a PVC rack with back heat

I just bought my first tub rack.

It's a 5 slot, 32 qt PVC rack from Boa Master.

It uses 12 inch heat tape down the back.

I currently have the thermostat set to 100 degrees and I'm reading about 86 degrees inside the tub.

What is the "typical" thermostat setting to achieve a 90 degree hotspot on the floor of the tub?

And is this even possible using back heat?

how long has it been running

it will be different for everybody because peoples rooms are different temperatures. Where is your thermostat probe, and how long has it been running?

I've been adjusting since Sunday, but it's been running at 100 degrees since yesterday.

The probe is taped to the heat tape in the middle of the tape, it's 3 feet long.

I guess the question is, is it realistic to expect the bottom of the tub to get to 90 degree as you would with a UTH, using back heat tape?

how did you secure the probe? with Aluminum tie? if so that might be your problem, as aluminum tape acts like a heat sink and will affect your probes readings