I have a no name Eagle Rock knock off from Enco, to get good knurls I had to take the sloop out of the longitudinal axis by tightening the center screw and nut. By doing that I was able to stop most double track issues. That and making darn sure my rig was dead square to work and upper and lower arm were dead center vertically. I have an acquaintance, late 70s age, owned his own shop for most of his life, he never contemplates diameter he just knurls, either bump or scissors style, makes pretty good looking knurls, says if he has trouble double tracking he cleans the knurls, uses more force and does em again. He just rolled his eyes when I tried to explain the calculations for proper starting diameter, saying something about why change what works for him. I myself am not so lucky, but I had no argument so left it alone.