at the very least you need to measure the temperatures in your enclosure with a digital thermometer. an accurite (which is available at walmart for about $15) works well for this. it allows you to measure temps on both the warm end and the cool end. measuring the temps by feeling the bottom of the cage isn't accurate at all--reptiles are much more sensitive to heat than humans are and what feels warm, or okay to us could burn a gecko that lays on the same spot.

using a uth without a thermostat will not provide consistent temperatures and you have no way to keep the temperatures from spiking too high or too low. at least get a dimmer that you can hook up to the uth, that way, if you see on your accurite that the temps are too high or too low you can adjust. this is the cheap way to go and does require a lot of monitoring. better to just get a thermostat, set it and you are good to go--no worries that your temps are all over the place.

i personally wouldn't even consider using any kind of heating element on a reptile's enclosure without a thermostat--why would your risk injury to your animal(s)? it's just one of the things you have to factor into the initial cost of your setup. if you can't afford it, then you need to wait until you can. here's a link to some great thermostats: http://www.reptilebasics.com/store/home.php?cat=250. the johnson or the ranco (pre-wired) will work fine for a leo setup.