The J T entity posted thusly:

What I did was drill a hole in the middle of the table, for the bit
to go thru.

OK, but I am trying to attach the router to a table base (Freud
Universal Table Base). I guess you might call it an insert. It
requires a rectangular hole in the table top, with a step/ledge. I
managed to mount the base on the table.

There are two problems with the next step.

1. The table base has a 1 3/8 hole in its smallest insert (it has
two inserts, one inside the other)
2. The router base has a central hole somewhat larger than 1 3/8
(I don't know how big. I didn't measure it).

I did not wish to buy a Skil guide bushing set, as I already own a
Freud set, which fits the table base insert. So, I can mount a Freud
guide bushing, and can then chuck a bit in the router to approximately
centre it (have to eyeball, because the IDs of the guide bushings are
not an exact fit for any of the bits I have that would be suitable for
centering (ie. pilot bit).

Then used the router base to lay out the screw holes.
Drilled, and counter sunk the holes.

So, I then mount the router, with the bit sticking through the guide
bushing, and get set to drill the holes. Oops... the router base is
not visible, so I have to take the router out and leave the base in
place in order to use the router base as a template.

Well, I tried that using tape to hold the router base to the table
base, with the result that the bit rubbed on the guide bushing when I
reassembled everything. It was close, but I don't think it was close
enough.

After following the suggestion to use pan-head screws and a flat
bottomed counterbore, I finally got the darn thing centred. Now I am
going to try making a base for the plunge base, using some Lexan I
picked up today.

I used 3/4" plywood for the table
top, so did have to take a bit out on the underside, but that was no
prob - too thick otherwise, should have cut a lalge hold in the 3/4",
then topped with 1/4", and put the screw holes in that. But, didn't
have any 1/4" on hand, so just went with what I had. Anyway, just
screwed the router in. No prob.

Well, that sounds easy enough, but of course I ran into problems you
didn't, because of the weird hole size in the router base, and my
requirement to incorporate guide bushings.

I didn't measure any of this, just eyeballed it. Would have been
simple enough to square out tho. Find the exact center of the table
(just go from corner to corner), mark it. Then reference the screw
holes, et all, from the center. No prob.

If I'd wanted the router so I could slip it in and out of the
table, I'd probably have gone with 1/4" ply over 3/4". Square base on
the router, sqpare hole in the 1/4". Smaller, round hole, to take the
router, in the 3/4".

I do want to slip it in and out, mainly to change bits and guide
bushings. For hand-held work, I will swap the motor to the plunge
base, which will have the Lexan on the bottom.

The router bit hole is maybe 1 1/2" to 2". The hole size is no
prob.

Well, 1 1/2" to 2" is definitely not what I need. If it was, I could
tolerate a few 32nds in centering. I need a stepped hole, accurate to
a 64th or so, and the router centered to that.

What's really fascinating to me is, I didn't ask anyone anything
about how to do it. Just looked at the router, and decided how to do
it, did it.

That's what I tried. It didn't work worth beans, so I thought I'd find
out how others would approach the problem. Looks like the right way to
go would have been to make a new router base first, centering it on
the router, then using that as a template for the table base.