The key is in this paragraph from the top post:
In other words, the cross section of the flue and that of the hood should be approximately the same, and the area of the opening should be about 65-70% of the flue's cross section.
So, if you're planning to use a 12" diameter flue (recommended), make your hood from one of the propane tanks, with a 10" square opening. If you are going to use a 10" flue (as I did here), then use the expansion tank and make an 8" square opening.