Before committing yourself to craziness consider this: all thermometers and thermostats (unless you are spending mega dollars) have error margins. Check the package of your equipment. Cheap thermometers are likely to be out of calibration with no way to adjust them. If each piece of equipment is off by two degrees in opposite directions then you have a 4 degree error right there. Also, how cold is the room temp (outside the rack). This plays a course in how hard the heat tape has to work. Some say 3" is fine for a hot spot but I disagree. I only use a 3" strip for my 12 quart tubs and a double 3" strip for the bigger tubs. Also, I only use the 10w 3" tape because the 6 watt seems a bit weak in the winter. I see variation in my rack as do you. I set my herpstat to 97 F. The thermometer taped to the heat tape always reads 96.6 F +/- a tenth of a degree. I have two thermometers in each tub. My warmest hotspot is 94 F and my coolest 90 F the cool side varies with the room temps but I never let it go cooler than 75 F. When the cool side drops I drag out the space heater. I think you should first test your equipment. Tape all the thermometers to the heat tape. Note each thermometers error margin. Unless you have a faulty herpstat (unlikely) or probe your tape will be the temp you have set on your herpstat. Based on what I have seen, you loose from 4-7 degrees of heat from the tape to the bottom of the tub (more if ambient is cooler). Were me, I would add another strip of 3" 10w to each shelf but many will say this is unnecessary.