UTH's use Wire inside run Zig Zag so you are going to get different temperatures all over it. What you have going on is correct. 87 degrees is a perfectly fine Hot Spot. As a matter of fact most ball pythons prefer 86°-88 and mine are in the cool hide 95% of the time at 76/78° nighttime & 80° daytime.
What you're doing is perfect and the fact that you have a part of the UTH under the substrate that reaches 90 is as high as you want to go. Dont worry everything sounds good