I'm with the others... you NEED a thermostat. Here are some things to conisider... in my opinion...

1. Your Heat gun. Are you shooting it through a glass/plastic/screen? While that shouldn't change anything, it surely can alter the temps. I feel the accurite thermometers are pretty reliable, so I'd go by that. Now, having said that, if you can post a picture of your setup, it will help everyone to see and give you recommendations. Now, How much substrate, and what type of substrate are you using? As said above, if your surface has to be 140 to get your substrate surface to 90, you've got too much in there. You don't need much. A lot of people, myself included just use paper towels, or maybe some paper. The thick substrate/bedding looks nice, but isn't needed or desired in most cases.

2. You can order a thermostat from Amazon for $40 or so and have it in two days. You can pick one up at a petstore, or you can order one. The herpstat line seem to be the best. Yes, they are pricey, but worth it. If you have to wait a few days, you can put your UTH on a dimmer switch (found at Lowes/Home Depot). That you can use to dial back to how much power gets to your UTH, and somewhat control the temp. Another option, and while I don't recommend this, i've seen it work for a short time... if you have a really good timer, you can set your uth on a timer. This takes a lot of fooling around and monitoring, as the temps don't always drop or increase at the same rate, so is still dangerous to the snake.

Just get that hot spot under control, and you should be good to go. Also, it is a good idea if you can get both hides to be the same.