Re: Need pics of how you have your probe placement
Personally, I would install belly heat in addition to the back heat. If you're not up for that project, you can try insulating the rack using either the styrofoam board found at Home Depot/Lowes or radiant insulation like Reflectix also found at Home Depot. Good luck!
My house is about 77 degrees most of the time. I have my rack set up with belly heat and the probe attached directly to the tape. I run a herpstat and set it pretty close to what I want to achieve. I set it at 90 and get a 91 degree hot spot in the tub measured with my IR gun. THe cool end as a result of radiant heat moving across the gradient ends up being about 80.
Ok now I am confused. You have 82/90 currently? What is the problem? Are you trying to drop the max temp of the flexwatt?
I too have read tech info from Calorique that stated not to run their product over 110 for extended periods. I have not tried to use back heat my snake room is quite cool and I have needed to insulate the rack and run 4 inch and 17 inch to hold proper temps.
I suspect that if you wish to drop the flexwatt temp you will need to use belly heat. I have my temp set to 94 and get 91-89 in the rack (different levels) I would suggest that a fail safe is the best option to insure the safety of your collection. You might also look to different wattage flexwatt to further increase safety margin the lower wattage's for a given size have a lower max temp. My rack unregulated produces 100/87 this is scary high temps but is tolerable for the snakes for a short period of time.
Personally in my racks with back heat I like to have an empty tub with my tstat probe in that tub. This gets my tub temps at what I set the tstat to. I do have a space heater to heat the room also. I have boaphile PVC racks...