Wheel Truing Stand Alignment Tool

I recently got myself a cheap(ish) truing stand for building bicycle wheels. While the thing isn't the highest of quality, it does substantially do the job. Out of the box however, it wasn't particularly well calibrated. I tried fixing it with a square and a steel rule, but this proved to be difficult, needing at least three hands. Regrettably, I only have two.

Instead therefore I've made this tool which holds a steel rule centrally between an axle. Obviously my steel rule is unlikely to be the exact same dimensions as yours, so I've included the f3d file so you can edit the parameters to suit.