I've tried just about every position possible and honestly there is no right answer. It's about the same as asking a hitter where he starts his hands. No matter where you start you need to put the pci on the ball to hit it consistently. Starting low may feel like it helps if you are consistantly dropping the pci but you are kind of supplementing for something you arent good at rather than directly attacking the real issue. Use practice tools efficiently, dont go in there and just try to hit the ball, focus on what you know to be your issue and really work on getting better at it. For instance, dropping the pci is largely an issue with people twitching or being jumpy when pushing the swing button. Go in to practice and try relaxing your muscles to eliminate that jump or twitch and the majority of the time the dropping of the pci issue will resolve itself, repeat this and build good habits. End of the day your ability to put the pci on the ball isnt largely affected by where you start it, there are top players who start in all different spots and there is no right or wrong way. I'd stress just to not start somewhere that feels uncomfortable as thags just starting you in a bad situation pre pitch.
That was super long winded, to answer your question, I start middle. I move the pci around before every pitch but let it rest middle just before release.