Don't try to get to the thickness of the plywood.

Cut a rabbet to a "known" thickness(1/2" is good)
and then route a slot using a 1/2" bit.

Using this method, you can really get very close
and not worry about weird sized plywood.

Actually, I would make my dado and sneak up with
the rabbet.

You can do this with a table saw or a router table.

mkr5000 wrote:
I really need to route it because I need to plunge into it and don't
want the slot the total length of the board.

Been a long time since I've done this, so can't remember if I can
route that wide a slot (3/4") -- just 1/4" deep or even less, just
enough to fit another piece of 3/4" plywood into.

Do they even make a 3/4" bit or would I have to do multiple passes for
both the width and depth? (yuk).

thanks