I've been doing the series once a week for the past two months (ish), and even now as I progress through the calf raises (which I reduce reps or add a bit of rest time in as needed) when I go to the seiza's my calfs want to cramp up because they are in the shorted/compressed position for two minutes after.
Is there an alternate series of the stretches to allow for the calf to relax a bit more between these two? like doing the calf raises with the kneeling Achilles, or just keep the reps reduced to not push them as hard?
We instruct a stretch class at our gym and I notice other folks having the same issue.