I'm getting close to finishing up my PR TN Mt rifle and I plan to make a sling out of upholstery webbing (2.5" webbing, doubled over with the open edge sewn shut). I have one sling swivel mounted through a stud on the toe plate and the other through an underlug just a bit forward of the middle rr pipe. The linear distance between the two swivels will be about 38". Is there a "rule of thumb" for determining how long I would make the sling for comfortable over-shoulder carrying? Or is it just try-and-fit?