Hi Johnathan

personally I wouldn't go for that length of travel on unsupported rail. The calculation you need is for a cantilevered beam, just you have it mounted vertically. As a rule of thumb I look at third points, so for a cantilever of 400mm you would need a 1200mm rail with the mounts 800mm apart. (not very feasible)

If this is a router then I guess this will then be mounted on a y axis gantry and needs moving as well so overall weight will be an issue. Is there a reason you need such a long z or could you just change the bed height