I used a repti-temp 500R on my inc last year and got away with it, but went and got a herpstat for this year.
I was constantly watching and adjusting the repti-temp to keep "somewhat" stable temps.
I placed the t-stat probe behind the flexwatt, some people hang the probe in the middle of the inc in the air.
The only temperature I am concerned with is in the tub, so the only thermometer probe I use is in the tub.
I also used hatchrite last year and learned that it quickly dries out in a non-sealed tub.
I used the same tub in your picture but didn't use saran wrap on the top and after 2 weeks my hatchrite dried right up and I had to replenish water over and over for the remainder of the time in the inc.