James posted a link to a video recently showing how to make a router guide for cutting grooves. Maybe if he sees this he will repost it. It is a thick piece of mdf or plywood which acts a fence glued to a thin piece of plywood which is wide enough that you have to cut a bit off when you make your first router pass. This shows where your bit will cut. (You have to keep using the same bit every time of course). All you have to do that way is to put a piece of your plywood edge down on the piece you want to rout and pencil mark it. Place the guide on one line and cut then put the guide on the other line and cut that side. Doesn't matter what your plywood thickness is, you will always use the same bit. When you make the guide use a factory edge for the side the router will butt against. The other side doesn't matter.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.