Here I'm scribing a mark on all my shelves using a height gage. Now I realize that most woodworks don't own a tool like this, but I happen to be a machinist and don't know any other way to put a mark on a work piece that would be this accurate. Again, this is overkill, but I happen to own these tools so why not use them.
 

This should give you a good idea how I'm lining up everything. It's a little dark inside this area so I'll be using my flashlight and magnifying glass to make sure I'm lined up correctly. This worked just as I envisioned it would, so I'm happy so far.
 

Here is my new router bit that I'll be using. It's 3/4" diameter with a top bearing. Most template router bits have a bottom bearing but I couldn't use that kind with this fixture.
 

Well now...that's not going to work like I wanted.

See how the bearing is right at the top of my fixture? And see how much the router bit is sticking out from the bottom of it? I want to take 1/8" deep cuts, not the entire cut all at once. I'm not going to use it like this because it would take too big of a cut, so I need to come up with a fix here.
 

I did some measuring and I need 3/8" more material added to the top of my fixture to fix my problem. I found a piece that would work so I put it through my planer and brought it down to 3/8" thickness. Then I cut the width and length to what I needed.