Would not you want to stretch the tendon, as well as the calf muscles, to insure a better all around stretch to the calves themselves? Though some guy's like the bent knee version (and for the most part, use way too much weigh and bad performance to do the calves justice), you can deflect some of the direct calf motion that way (muscle stretch/flex...ROM). Might also want to work the calves one leg at a time, as you would ham curls. Good Luck.
Side Bar: might want to try 40 yard sprints as part of training to really blast the calves, from time to time..