Thanks for all the responses. I dug into this a bit more and did some calculations (I can send the diagram if anyone is interested) and determined the following equation:
For the first cut to make the female template from the male baseplate the offset from the plate is:
D = B/2 + O/2
Where B is the diameter of the bit (divided by 2) and O1 is the diameter of the guide bushing (divided by 2) with D being the distance between the baseplate and the (new) inside edge of the template.
When routing the hole in the table using the newly created female template, the offset equation changes to
d = r/2 - b/2
Where b is the diameter of the bit (divided by 2) and r is the diameter of the bushing (divided by 2). Since this is an inlay type operation where the final inside cut needs to match the original baseplate, these two equations need to be set equal to each other to determine the bits that are needed and the guide bushings needed.
So,
D = d
which means
B/2 + O/2 = r/2 - b/2
If you assume that B = b (the same bit used in both operations) then the equation reduces to:
B/2 + O/2 = r/2 - B/2
or (after some algebra)
2B + O = r
Ok, so there's the equation. If you have Excel you can quickly make a table of all router bits (straight cut) and guide bushing coller sizes to determine the values of "O" and "r" that work. There are probably only a few.
For example:
If you have a 1/4in. bit, and a guide bushing "O" of 1/2 in. for the first cut (to make the female cut) then you'd need another bushing "r" with a diameter of 1in.
r = 2*1/4 + 1/2
r = 1/2 + 1/2
r = 1in.
So there's a combination that works (for those that don't want to do the math
. There is at least one other with a 5/8 in. bit that works (from the Router book by Spellman) but I don't remember the bushing sizes. It's explained in that book pretty well, but only for that particular size of bushings and bit. If you have a different sizes it doesn't explain how to determine what you need. Hopefully this will help if anyone has different sizes of bushings and bits. BTW, all of the bushing sizes are outside diamters (don't use the radius values if those are given!)
Hopefully this will help anyone else doing this.
Thanks,
T.