I've never made a string for mailorder, but I can guess that it's fairly simple to keep a notebook on particular manufacturers strings and how to build them. I've noticed all the strings I've bought in the past, had more serving than needed. It's probably common to add a few inches to the minimum amount needed, to cover any unexpected variables with a given bow.

One reason I like to make my own, is that I can stop the serving exactly where it leaves the cam. I don't care for the look of lots of serving, though some people do. I've had friends who wanted the serving to stick out a couple inches when I made a string for them.

I must admit, the first time I made a cable for a roller-guard bow, I didn't like the results. The serving covered most of the pretty cable I just made.