I got Spielman's "Router Jigs and Techniques" and a sheet of 1/2" acrylic then sat down to make a few of the custom sub-bases he shows in his book.
For acrylic I figured I needed to get the mounting holes pretty close to exact in relation to each other and waste as little acrylic as I could in the process. The problem is that the mounting holes on the Porter Cable 690 series (and possibly other makes and models - I don't know) are arranged in an equilateral triangle with the centers 4" apart from each other. That's great and all, but I needed to know how far those screws are from the center so I could lay it all out.
I broke open the absolutely most useful book on the planet (Pocket Ref - Thomas J. Glover) and fired up MS Excel then sprained a few brain cells going through some trigonometry to get the answer.
That answer is 2.309".
If you have a router with a 4" triangular mounting hole configuration and want to make a new sub-base, here are the steps I used to lay out the sub-base:
Mark off a square slightly larger than the radius of your router base on the corner of your base material. Draw an X to find the center. Set your compass to a radius of 2.309". Draw a circle with the pivot point at the center of your square. Move the pivot point to some point on the circle and use the compass as a divider to walk off and mark 6 points on the perimeter of the circle. The seventh mark should fall exactly on the first mark. If it doesn't, make a small adjustment and start over. Three of those marks will intersect the circle exactly where you need to drill for the mounting screws. Drill and tap (if necessary), then mount the router to the sub-base material. Now would be a good time to trace the outline of your base and cut it out. Insert a center cutting bit (V bit), fire up your router, then lower the bit until it just scores the material. This mark is exactly where to cut/drill to make the center hole. If you wanted to be real spiffy, you could drill a small hole at the center mark and put it on a pivot pin the rotate it against a sander. This will give you a perfectly round base with a perfectly centered bit and eliminate the concentricity offsets that may be encountered by using the factory base.
The same general technique would work for making a base for a router table.
For different router bases, here are the magic numbers based on the distance between mounting holes:
2" 1.155" 2.5" 1.443" 3" 1.732" 3.5" 2.021" 4" 2.309" (PC 690 series) 4.5" 2.598" 5" 2.887" 5.5" 3.175" 6" 3.464"
Here are the radiuses you would use for locating the mounting holes on routers that use four mounting screws arranged in a square. The column on the left is the distance between two adjacent mounting holes:
2" 1.414" 2.5" 1.768" 3" 2.121" 3.5" 2.475" 4" 2.828" 4.5" 3.182" 5" 3.536" 5.5" 3.889" 6" 4.243" (PC 7518)
I thought I'd post this info on for the archives. Hopefully it will come in handy to someone someday.
Good luck! -Rick