To your question .665 is the inside diameter of the choke. The more you constrict the tighter your pattern will be. Typically your tightest chokes are around .660 for 12ga. A tighter choke doesn't always mean a better pattern though. The choke/gun/shell combo will make a lot of difference in pattern density.

Don't over-think it though. Get a .665 Jellyhead and some good shells.