Definatly sounds low.I would think 6 and 3/4 to 7.5 would be in a good range for a 3 piece longbow typically.
It wont hurt to overtwist it in my experience to a point.I have shot strings that had too many twist in them.Or more than I would like but they were functional.
Perhaps its not the original string made for that bow or it stretched alot over time.
I would add more twist and see how it goes.It will shorten quickly when its got too many twists or an undesirable amount of twists.
sounds like you may want to get a string made soon of appropriate length.
Brooks made some pretty bows!