If you are shooting a 30" cam and dropping the string to the 29.5 position, then yes you WILL lose peak weight...a substantial amount depending on the individual bow.

By twisting the cable back up to regain poundage (or to get the cam in the proper position) you are increasing the draw length, hence you are running a round in circles.

If you have a 29.5" draw and want to shoot 70 pounds, I would suggest you get a 29" cam put on your FX, and use a longer string loop (or put one on if you don' t have one) and/or a different release head that has greater distance beetween the head and the trigger.