Many people don't feed medicated starter at all. The theory is that if you get an out break of cocci you treat then rather than attempting to build immunity.
Amprolium is a thiamine blocker and the science is in, a thiamine deficiency is the main contributing cause of wry neck. Check the bag for ingredients .
I keep a tub of amprolium in my first aid box because even after they have had medicated feed for the first 6 weeks if you have a cocci outbreak chances are they will be just as susceptible to it as the older birds. It's important to remove the medicated feed as soon as you see symptoms.
Routine vits ( as you are already administering ) and additives high in thiamine and niacin will go a long way to preventing wry neck.
Brewers yeast, vegemite and marmite are also good sources, that most of us have on hand.