I hope this will get somewhere
Actually, we have already gotten somewhere. The method I presented will get you in the ballpark and will work quite well. We are now closing in on home base. This is both good news and bad news. The good news is that with precision we can give you a length that will result in the absolute lowest temp. The bad news is that, given such precision, if your load is a few extra watts your temps fall off a cliff. We will probably want to go just a little shorter than the precision length so that you can have some room for extra wattage.