Sounds like a good find. I think 172 is a little fatter than my 500. 170 is what I usually run. There is a jetting chart that is found on a sub-menu on the main page that is very accurate. Also FMF has their own recommendations. If they both are the same, you really will know the answer. I think the FMF just adds a step fatter on the needle adjustment from stock and the Pro Circuit goes leaner a step. Of course, if its aint stock, it's only going to give you a starting point. It will probably sound fat in the garage idling before it gets warmed up and cleaned out. Jetting for the main is changed when its running fat Wide open. You are thinking the idle jet is fat, but I would wait to see how it acts after riding til cleaned out good.