420 rail length is dictated by point x measurement and staircase angle, these measurements are used to decide the amount of rail overhang over the top nosing to allow the installer to set the footrest height and to reduce the tripping hazard on the landing
it use to be found in the back of the paper installation manuals, always cut the top of the rail, never the joint! if you are selling it measure the total rail length and add it to the advert, then the onus is on the new owner to get it cut and installed correctly…….
