I just use my own arm span length (mine is approx. 5.5 ft). When doing a one piece, i know the total length from various online sites, and then just use the length of a racquet head for determining the short side (ex: a 16x18 string pattern would be 8 lengths (half of the lengths) of the racquet head and another 3/4 length for pulling and tying off).
I made my own using a 1x4x35 inch piece of scrap lumber. Notched a little at both ends and marked at 11 and 23 inches. Most all my jobs require 34-37 feet of string. Each revolution is 6 feet. Easy Peasy.
Next to my stringing machine I made a gizmo to hold reels as I pull string. I bought a pretty nice work bench and have that behind me as I string and I made 3 one foot marks on the top of it from the right edge. When I measure string I hang the reel on the gizmo I made and measure out against the marks on the work bench.
I measure the string using the head of the racket.
- assume 16*19, measuring mains:
- first pull ~6m from the reel, keeping the end in my hand (and keep it there till the cutting of the proper length)
- measure 5 length of center mains
- measure 3 length of outer mains
- add 20cm
- double and cut