You were right to put the feet on the enclosure. A little air flow under the UTH actually helps it work properly. Just to be clear, a UTH must be used with a thermostat to prevent from cooking your snake.

You should be taking temps right on the glass under the substrate where the UTH is. Use a thermometer with a probe.

A hot spot of 92* (surface temp) is sufficient, but you should try to keep your ambient temps above 75* at least. shoot for 80*.