Woodpeckers Fat Head Hook Rule
features a positive physical reference locating the zero point of your rule exactly at the edge of your stock. It's the only hook rule on the market where your rule lies flat (all the others require you to hold the rule upright). Don't fumble with aligning the end of your rule to the edge of your stock. Just use a Fat Head and you'll know for sure that you're right on. Even if your stock has a round-over on the edge (like standard framing lumber) you're always calibrated perfectly with the primary edge.
This rule is the 300mm model. It is also available in 150mm and 450mm.