I have found, through using quite a few of those rheostats, that each one seems to work at a slightly different level. One of mine will keep the temp right around 94, and another will leap to 102...both on the absolute lowest setting.

If money is really tight, as it is for me right now, and you can't afford the $75 for a Ranco or Johnson yet, then I would seriously consider picking up a ReptiTemp 500R for the meantime. They aren't reliable at all as a permanent thermostat, but they work a lot better than those rheostats until you can get the 'real thing'. I just ordered a second reptitemp for the corn snake tanks since the one I had on them is being used for my rescue BP tank.