Sizes for UTH

There's a few different sizes that UTH's come in, obviously. I have a Corn Snake in a 20 gallon long, and my lights keep on giving me a hard time. I've decided to go ahead and purchase some quality heating equipment.

got me a Reptitherm Heat Pad and a Spyder Thermostat to regulate it. The pad is 11 x 11, and the cage is 30 X 12 x 12. My question is if the size of the heat pad is too large? Most things I've read say to only cover about a third, but this is a little more, but less than half. I just want to make sure everything is all good, and so I don't cook my Corn.

It's close enough, 1/3 is just a guide. I'd be sure to have the heating pad on some sort of dimmer switch and use a thermometer to check the temperature of the hot spot.

A third would be 10x12 yours will be covering 11x12, one inch will not make a difference.