You adjust using the scores on the rails. Grab a set of small hand pliers, clamp them down on the rail, and bend the rail just before the desired indentation size. Or I’ve done it by hand while using a desk or table corner. I’ve never experienced any difficulties. Instead of clamping your pliers above the scoring, clamp them just before the score mark you want to reduce the size to. Lay the rail out flat on a table or desk. To shorten the rail, slide it just a little bit off the table. Holding the remaining portion of the rail on the table with one hand while bending it downward with the other using a hand tool.