Ideally you want the belly hot spot to be around 90* and the ambient temps to be in the low 80's. high 70's. The air doesn't need to be 90* as long as there is a belly hot spot of 90*
I recommend setting it up this way. Keep your thermoSTAT probe outside the enclosure so the snake can't move it and give the thermostat a false reading. Taping it to the outside of the UTH works. Now put a thermometer probe underneath the substrate right on the glass where the UTH is. This will tell you the hottest point the snake can reach. Make sure you use a thin layer of substrate so the heat actually gets to the snake. Set your UTH until your thermometer reads about 90*. You might have to make a few small adjustments before it gets perfect.
What are your ambient temperatures without the lamp? If they are around 75* and above you don't need the lamp at all, if they are lower then that then you might want to either get a lower wattage bulb or a dimmer for the light so it doesn't stay on full blast 24/7.
It's tricky to find that happy medium, but I believe it is possible with what you have.