depends alot on the bow & cam design.

BowTech's singles, and newer Mathews HP cams will lose/gain roughly 1FPS for about every 3 grains of weight from 5 grains per pound up until about the 6 grain per pound threshold, then the ratio gets closer to a 4-5 grains per FPS.

BowTech dual cams often have a 1 FPS per 2-3 grain weight ratio up until around 6 grains per pound.

The Hybrid cam bows (C.5, CPS & copies) I've owned seemed to be somewhat the opposite...not picking up as much speed as other designs with light arrows, and not losing as much with heavier arrows. they maintain a more steady ratio throughout the range of arrow weights.

There's no cut and dried formula these days as cam efficiencies and designs have changed alot since the old 1/5 ratio was brought up.

I would guess you would lose 7 or 8 FPS assuming a typical single cam bow.