Rack heating/thermostat question

So I recently built an 8 level rack for 12/32 qt tubs. It's made out of 1/2"pvc and has open sides. I purchased a vivarium-100 (before I realized it was pulse proportional/thought at vivs were) and placed the probe on level 4 directly on the heat tape. I've read a couple other post where people have to set the thermostat higher than the temp we are needing to achieve the hot side norms. I've had to set mine at 104 to reach 89 on level for. Using a seperated zoo med thermometer to read temp. Is this normal? It seems like I'll have to go to 108 to reach a 92 degree warm side. What do you guys place your thermostat at?

(Rack is empty till I get it perfect)

I think I figured it out lol...I had a piece of aluminum tape directly over the probe. I changed it to directly behind the probe and now set the thermostat to 98 to maintain 93 which feels a lot better!