The position of the grooves depends on everything: the length and angle of the lug on the change lever, the length of the arm on the gearbox, and the angle between the gearbox and the change lever. Not to mention the gearbox internals.
I think you'll have to put the lever you're going to use on the pivot bolt, set up the length of the coupling rod you want to give the angle of the quadrant you want, then mark out on a template the point at which the various gears engage and disengage. The various grooves and shoulders will then be correct for your bike. Locate second half way between where the gear engages and disengages if you can't see inside the box.
Good luck.
Leon