Is my UTH too small?

Hey everyone. I have a 36x18x18 tank, and the UTH I got for it was the Zoo Med ReptiTherm 12x8 UTH. Now that I have been assembling my setup, I'm wondering if this is too small. My warm hide is a good temperature, and there's a small hot spot with a good temperature on the floor of my tank. But is that small hot spot enough? The warmest place is pretty much centered in that half of the tank, and toward the edges it gets considerably cooler. Should I have gotten the 18X8 UTH? If I replace it with an 18x8, should I install it the "short way" (with the 18 inch dimension going front to back along the 18 inch dimension of the tank) or the "long way" (with the 18 inch dimension running from the left wall to the center of the tank)?

Thanks!

Based on your dimensions that should be good. That's 1/3rd of the tank covered. Just make sure the uth is secured to the bottom of the tank completely touching the surface.

It's big enough.