My guess is that you don't have to do anything. Half of the enclosure has higher access to humidity because of the soil type you are using and the hide with sphagnum moss. One half should be very low humidity for a sulcatta and other desert varieties. With that said, how you measure humidity...
Make sure bulb is on one end. Your tank is tall and thus hard for heat to dissipate. Lots of inexpensive options when they are tikes. I like the cement mixing trays you can get at Lowes. Also, consider soaking every other day but in tepid water and for a period of about 10-15 minutes.
Your current methods seem fine. Micro-climates are the way to go. Look at videos offered on Garden State Tortoise and Arizona Tortoise Company. The prefer fairly low air humidity but early morning misting mimics the fog effects where they live. The higher humidity and temperature variations...