Assuming the calf weighs 180 lbs at weaning taking to 400 lbs = seeking 320 lbs of gain.
Target of 2 lbs of gain per day = 320 lbs in 160 days.
You should limit grain to 1.5% of body weight which is an average of 4.35 lbs of grain per day.
4.35 lbs x 160 days = 696 lbs aka 700 pounds.