Try lipstick. If your vanes are hitting it will leave some on the vanes after the shot. If you have been shooting them for a while it should be pretty easy to see any way. There will either be marks on the vanes, or on your rest where they are hitting. Look at your rest to see if there is any color that matches your vanes. If there is you have contact. Same thing with your vanes, if you have black covers on your prongs they will leave a black mark on one or two of your vanes.

If you want to calculate your FOC here is a great site.

bowjackson's

Paul