http://www.loc.gov/pictures/resource/ppmsca.36462/ http://www.loc.gov/pictures/resource/ppmsca.34358/ http://www.loc.gov/pictures/resource/ppmsca.31450/
I wouldn't tie a knot, but cut the strap and sew it up properly. I believe the standard strap length was 6 foot long. When a soldier had his full pack on, either a blanket roll, or knapsack, the canteen would be the last thing to go on, the strap needed to be long enough to go over that equipage so he could access it on the march. Now, I have seen pics of Cav folk with knots tied in the strap, to hang off of their saddle.
This picture is my smooth-sided canteen with a 6ft strap my Wife made for me out of cotton duck canvas, it's the perfect length for marching.
Agreed......cut and sew the strap......It should, just about, hit the top of the hip/butt....But, there is no "Regulation" for the suggestion that I just gave!!....The best thing to do is shorten it to the point where it is comfortable on you and allows easy access when on the march and/or carried with other gear.....