most calmers are based on magnesium, they will only have a calming effect if the horse is deficient in magnesium. Instead of buying branded horse magnesium (which you will pay through the nose for!) I would buy it online as plain magnesium. Ours are both supplemented magnesium as our grazing is deficient. I get it from here (cheapest I can find, lots of useful info too). http://www.naturalhorsesupplies.co.uk/p/category/0802204936-Magnesium+Supplements/
I get the heavy magox, lasts for ages!
Equine America Magnitude seems to work very well on one of my horses who can be a little bit highly strung at times and over reacts to things and gets himself in to a bit of a pickle. Another one that has been highly recommended to me is Nupafeed, although is more £'s.