I haven't heard of using Ace orally. Not sure if you can or not.
Here are some MAJOR tips though
1) Never give the ace once he/she is already worked up, it can be less effective or might never work at all
Give me your horses weight and more info on behavior and I can probably help you more, however almost all sedatives come with instructions. Ace is a good brand. I'm all for natural stuff but any natural sedative I'vee seen doesn't work as well or for as long. The Ace effectiveness depends on your horse and its weight.
UPDATE-- Give it to the horse no less than 30 minutes before you need it to take effect if you are going to do it orally. 45-30 later it will take effect. Their should be instructions for your horses weight and size.
It can last from 1-4 hours depending on amounts and the horse. Hope I helped.
Update 2- I haven't used Ace in events. My barn manager uses Ace on the studs when we hold events out there to keep the quiet in their stalls. Most of them are but 3 of them aren't calm horses. They never seemed slow,they stayed alert, they just stopped kicking and throwing a fit. I'm sure the natural ones will be a lot less risky though, Ace might make her stumble at a high speed. We just walked and shown our studs in hand and they were fine. They were administered knowledgeably too, I wish stupid people would shut up.