For just a 900 who bolts and breaks loose, I'd give a good 20 ml (aka 20 cc) and check her reactions. If extremely groggy, lower dose. If still spooky, increase. Could take as soon as an hour to work or as long as 3 since she'll have to digest.
In the mean time it'd be worth teaching her she cant escape the halter by tying to a sturdy fence and using the stick / a rope to touch sensitive areas like the feet and underline while standing where she cant see you. Not hard or fast, but enough to make her decently uncomfortable. If she's drugged up too, she probably wont have the strength to really fight and will eventually act like it's no big deal.
If melatonin dosnt work, I've have good luck with tryptophan supplements.