Your rack is too HIGH.

Physic 101 : Remember that hot air go up and cold air goes down.

Meaning the heat used to heat the bottom move up and heat the other tubs.

Also, I think the heat mat are less constant than the flexwatt tape. I was using heat mat when I was younger, and I always had difference in temperature when I was switching them. They tend to lose their heat capacity when they get older. But I still have inconsistent temperature on my flexwatt tape, from one end of a shelf to the other end. Not a lot, but still.

One suggestion would be to build rack horizontally instead of verticaly. Let's say, instead of having 6 rack of 12 shelves high, you build 6 rack of 2 shelves with 6 tubs per shelves. And you uses 1 thermostat for each like you would. That should help you get a consistent hot spot.

If you are using an Herpstat Thermostat for your racks, I suggest you go with a PRO version or a model II. They have 4 probes, each being on separate line, meaning that if one broke, the others are still running. And a pro cost less than 4 basic, and you have more option in them.