I think it might be a 'seat of your pants' sort of thing. You might get close with the spacing but it might be hard to get exact.

Idea: Take a little carbon paper and rub the top of the existing nut then take a blank piece of paper and lay it on top of the now inked nut - hold paper firmly in place without moving it - press down over the paper on top of the nut to get an ink transfer of the old nut onto the paper.

You should be able then to match that to your new blank and get things pretty close.

If you file the slots too deep a mixture of bone dust and super glue can be used to re-fill the slot and you can start again.

Also, if you have a saw etc. you can just go to your local butcher shop and get bones for free to make nuts with.