I do not have a NuWave but I have an InfraChef. this is how I worked out my method.
When I first got mine I put the extender ring on and then set up the racks (2) . I put my thermometer on the top rack, dialed my temp to @ 270 degrees and then set the oven for 8 min. I watched the temp the whole time. It slowly reached 270 by 5 minutes time and maintained it. This is on the top rack. Then the oven turned off and I watched the thermometer until it began to fall. It was exactly 3 min more before it fell below 265-270. So then I did the same run with the thermometer on the bottom rack. Set the temp at @ 270 on the dial, set the time for 8 minutes, and then watched it thru the cycle. This time it was interesting. The temp did not get to 270 until about 6 minutes in the cycle and then after the oven shut off it did not cool down for more than 5 minutes. Then I ran another test run for the bottom with a test head on the bottom rack and a set of test limbs on the top rack. I also included the thermometer on the bottom. Set it the same way 270 for 8 min. This time the bottom reached temp at 6 min, and the top at 5 min but it still took it about 3-4 min to begin to fall temp wise.
I deduced that I needed to set the oven on the 270 for the 8 min, and then after the cycle finished leave everything shut for another 4 minutes to make sure that the heat was enuff. This method has worked well for me everytime. If I do not use both racks I adjust accordingly.
HOpe this helps. DO NOT experiment with your good stuff...use old parts so that you if get it too hot etc you will lose nothing. Onece you get it all working I think you will love it!