Mark, I went back and read your original post, and, although you're new to the forum, you seem to have a pretty advanced grasp of the process. That being said, since you are 'going where no man has gone before', just to be sure that your recipe is in the right ballpark, could you post it?
Ebay has used IR thermometers with auctions ending about every couple days. I got mine for $18 shipped, but it took me a few weeks. I recommend bidding $20, waiting to see if you win, and, if you don't, bid $20 for the next one. Eventually you'll win.
Honestly, you can try 500, but I really think 475 will get you closer to the 4 minute mark. It might even be 450. If 450=4 minutes, then 500 could translate to 2 minutes, so watch that bottom carefully.