I agree, stretch at the bottom and full contraction held at the top. I usually pyramid up in weight and will do long holds at peak contraction until my heaviest set and then use a short hold at the top of the movement. (heaviest sets will be for about 5-6 strict reps) I happen to have good genetics for calves so I train them once per week. I think for people with stubborn calves a progressive overload may be useful by doing them twice per week -- alternating between heavy days with big squeezes at the top, then next time doing higher volume with lighter weight and going for a full burn at around 12-20 reps.