Having got an aluminium remnant after machining the sanding disk, I though a router plate may turn out useful with jigs as well as holding the router upsidedown. So the plate had to ease mental arithmetic but also needed to be quickly and accurately screwed on the router.

The five millimetre thick aluminium left-over allowed three Ø200mm router plates and I roughly cut the shapes with a jig saw that worked fine with a metal blade at the very low stroke speed and moving slowly so that the blade does not heat. Then drilled a Ø8mm centre hole matching the shank diameter of a router bit.

That centre hole accommodated the pin of the circle fixture and after several passes with coarse then fine grits, I got three accurate Ø200mm circles. Making so much mistakes with calculator I now would rather simplify the base. This diameter can easily be divided by two.

I locked a bit into the router spindle then slid the centre hole of the plate along the bit shank and clamped, marked the four location screws holding the origin plate and released. Then drilled four countersunk holes and made some trials to get the right way of locking the plate against the router. Locking screws in the 1, 3, 4 and 2 order enabled enough accuracy to centre bit and plate. Finally the jig saw cut the centre aperture that didn't have to be absolutely concentric. The second aluminium plate sat on the router table extension of the combination machine and I kept the third for future use.