Recoil displacement is indeed the compression stroke multiplied by the piston/spring mass and divided by the rifle mass. In practice, the piston mass that contributes to recoil is nearer a third than a half. I think this is because the masses of the rifle and piston are highly dynamic during the stroke, as more of the spring wire starts to move and transfers more and more of its mass from the rifle to the piston. I believe that perceived 'sharpness' of recoil is a function mainly of the force placed on the piston at the start of the compression stroke, and am not surprised you describe your rifle as snappy, as the force on the piston at the start of the stroke is in the order of 830N (Newtons). By way of comparison, my TX200 with an 85mm stroke places a force of 650N on the piston, and I doubt anyone would describe it as snappy.