One important thing when using Flexwatt is to use the same wattage for the whole rack. Unless you use two different thermostats or something like a Herpstat II. If you mix watts on the same thermostat, You will get different temps. Say you put four shelves with 6 watt and one with 10 watt and all are running off the same thermostat, the 10 watt will run hotter than the other four with 6 watt. Now if you had those same four on one thermostat and the one with the 10 watt one a separate thermostat, you would be safe.
hope this helps,
Dave