I used a Reptitemp for my BP before getting a Herpstat, what I did was set the probe at the same place where my thermometer was on the glass, and waited till it got to 92F then turned the dial until the light came off. I noticed that the UTH kepted going up a few degrees after it turned off, it maxed at something like 94F.

I didn't read what substrate you use...
I used reptile bark, so I had to only use just enough to cover the glass to get the top of the substrate to ~90F.
Take note of the temps when the light goes on and off for a few hours, you'll need to readjust it a few times to make sure the temps don't go too high.

You can also use a smaller wattage infrared bulb for the cool side or use a Rheostat to get it to the temp you want.

I used a Reptitemp for about a month while waiting to get my Hepstat to come in (I'm in Canada and I had border issues). Let me tell you the peace of mind I had once I got the Herpstat was well worth the money.
Now my hot side stays within 0.3 degrees of what I set it.