Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
I have cracked open the door and pointed an infrared gun right after the 2nd panel...
If you do not have or have access to a donut probe or something to read the ink film inside the dryer.. you'll have to wash test to be accurate. Set up a black print, preferably something with detail and cut up some white shirts into squares, about 6-12 of them. Set the belt speed a little over what you think is cure... print and run test #1.. write on it with permanent marker what the belt speed was.Slow the belt and repeat until you've used all your squares. Wash them on heavy duty with rough clothes like jeans and see where the ink stops washing off.
You will still have to find that sweet spot in which the temp peaks for a split second. Also, does the temp drop at all when you open the door you quoted?
I cut it then print on each of the squares the same design?