Yep the calf/foot twitchies are the symptom that convinced me something was seriously not right. They started up 15 months ago and only improved with prolonged treatment, but they're still poppin along.
Scientifically, the lower leg is more affected due to over stimulation of a fight or flight response, and the calves/feet are a very stimulated portion of that response cuz we use them to run/use them constantly. Also have read the distance of the lower leg from the heart/brain etc plays a role. These are the most reasonable reasons I've read (and I've read a LOT).
And for me, all the Magnesium in the western hemisphere wouldn't touch these. For some the twitching is from Mag deficiency - if only I was so lucky!