They are supposed to be 1/2 inch from the shoulder seam.
I attached a 1/4 in. wide ribbon to the underside of my shoulder boards at both ends. Then I made corresponding small holes in the coat. [Use a bodkin, don't cut the holes.] The boards are attached with the ribbons.
This way the boards can be removed for cleaning. [It is also easier to change them if you ever get promoted.]
Michael "Books" Tatham
Capt. 5th Kansas Battery
Acting Artillery Commander, Dept. of Missouri