Sorry I can't help as I have never used anything to calm down my horse, other than the outing after the one I had been bolted with.
I am not convinced these calmers work. I think the calmer works as a placebo effect on the rider, i.e if the rider 'thinks' their horse will be calmer then there is a good chance they will ride it calmer and not tense up, get nervous, etc and transmit all that tension to their horse. I am not trying to belittle you, and can understand where you are coming from but it would be interesting to see if they have carried out a study regarding this type of thing. x
Why don't you try taking rescue remedy yourself instead?
we have tried couple of calmers and with each one still get the going bolt upright, napping and being a general twit so i'm pretty sure its not the rider.
I used it for the first time last weekend at a show - mare (new horse, don't know her v well yet) has been spooky and a bit tense, and the Carl Hester Pro Mag shots were on offer for 2 for 1, so I thought I'd give it a go - horse was much calmer and more focussed. It could of course have been just coincidence, but magnesium after all does counteract adrenaline, so I don't see why it wouldn't work. Problem with magnesium is that it is waterbased so leaves the system with sweat/urine, so only a very short term effect - though Nupafeed claims they have sorted out this problem with their calmer (which they may very well have done).
Ahem, sorry for essay - in short, especially with the current offer of two for one - I'd give it a go!