Strings that come with bows are notorious for not fitting right. You can twist it until it gets 2 twists per inch. Much more than that and the string begins to get bound up and loses efficiency. Check out SBD bowstrings, one of the sponsors of TradGang. If you decide to use them, call them the first time and discuss your specific bow with them. Or learn to make your own. Replacing bowstrings is a routine part of bow maintenance, and fortunately don't cost very much.
I would start around 8" BH with that bow. I doubt that you will want to go all the way to 9", but evidently that has worked out for some other people.