You are talking about the machine loaded type of standing calf raises, with pressure applied at the shoulder. Try doing one legged, with your toes raised (I use any machine with a floor stabilizer member, a bar that is 4 or so inches off the floor, narrow enough that I can stand on it with my toes and raise and lower myself. I hold a weight in the opposing hand and steady myself with the hand on the side I am exercising.
With a plate in hand, 20 reps is just perfect. This is one of the few that I stretch inbetween sets, else I cramp up. I alternative legs, and do 3 sets per side. Great calf developer. Use this alternately with the seated calf raise machine and with toe presses on the leg press.