This was harder than I thought it would be... Initially I did not have a protractor and just using the jig (which has sides at 60 degrees) was not as accurate as I would have liked.
I wanted the front mounts to line up with the existing edge on the board so as to save two cuts. After some trial and error trying to work out the best way to find the centre, I used the following procedure to mark out the board:
- Start off by using the protractor and a ruler to draw a line in from one edge just above the corner (about 31mm up from the corner) Draw the lin in towards the centre from the edge of the board at 120 degrees.( In the image on the left below its the line starting from the bottom right about 31mm in from the right )
- Position the jig in the right orientation across the corner and use the hole at the top of the jig to mark the center of the board along that line.
- Use the protractor to mark out the other two 120 degree lines from that centre point and draw them out from the centre
- Screw the Jig in place and align it with the angle and mark the holes and edges etc.
- Finally cut and drill 2 copies, one for the bottom and one for the top.
Update: I have created a modified version of the diagram from thingiverse that showed only one axis and have it showing the full layout. I found it more fool proof using a full printed http://www.billd.net/images/RostockBaseDims.pdf