It's not rocket science. Place the probe on the heat source. Now that does not mean stick it on the inside of the tank on the glass like you were saying was correct. Now here comes common sense. If you want to stick in in between the glass and the uth great. If you want to stick it to the bottom of the uth that's just fine to. As long as you can dial down or up the thermostat to get the desired temp inside the tank on the "hot spot" it does not matter as long as the probe is on the heat source controlling it.
I may have came of brash, and that's usually not my style. But it appeared to me that you had a answer to everything we were telling you on why you should continue doing it your way.