I used Equine America So Kalm with great effect on my box rested mare. It's based on l-Tryptophan as is Feedmark Steady Up, I was going to change her on to the Steady Up once I'd used my So Kalm, as it's cheaper, but she got turned away before I finished! I imagine it would work on a horse that So Kalm had worked on though. Magnesium didn't work on Maiden, and I'm told this is because she gets all the Mg she needs from her food (Mg supps work on the assumption that there is a deficiency in the first place which is making the horse tense).
So Kalm works within two hours (I think), it's best therefore to feed it regularly and not in one feed. Although I suppose it may have a built-up effect...? I have no idea, that one's a wild guess! It certainly calmed my horse down!