If your arrows arent already tuned to your bows, adding any broadheads will make your flight erratic. It has to leave the bow flawlessly with field tips in order to get flawless broadhead flight. Simply switching from hunting point to field point wont affect spine unless the grain weight changes. Generally speaking most of us wouldnt know if the weights flucuated 15-20 grains, just get them close as you can. Shoot your stone heads into blocks of closed cell cheapy foam, like the cheap coolers are made from. It wont hurt the edge and it will stop the arrow several times. I dont often shoot my broadheads because I know my bow and arrows are tuned and I know the broadheads will fly true as a result.