this is how i would set it up
rear: 6" shaft showing, 8" shaft compressed, 1" of coil preload= total rate of 57lbs/in ~ 14" 100/ 16" 150 =60lbs/in
Front 7" shaft showing, 9" shaft compressed, 1" of coil preload= total rate of 55lbs/in ~ 16" 100/ 18" 125 =55.5 lbs/in