I use Chamomile flowers and do find it to work.
I have never heard of the Tea recipe you posted....found that interesting, I may have to try that just to see. It sounds a bit more of a lengthy process then just throwing in the flowers though.
I don't do half a cup.....I feed 2/day a handful each feeding along w their regular grain ration, so if I were to wager a guess, me more like a 1/4 cup a feeding at the most.
I use it for my one mare who is much like the TB you describe...even though she is a WB, she is quite TB like in her characteristics....(gets that from her TB mom I suppose lol)
I also use it when when I know the horses may come into a stressful situation, such as at the show grounds. It just helps take the edge off.
Another herb you can use is lavender, it is said to do the same things as chamomile....I just find it is harder to get a hold of like the chamomile flowers
Horses are like chips...you can never just have one