Building exact width dado jig - choices...

If you use a bushing you have to account for the offset in the bushing which could mean trial and error to get the gap in your jig right. Using the bearing guided bit means you can use a piece of the board or panel you'll be using for your shelf as your setting gauge which assures you a perfect fit the first try.

You can build a jig that is two tiered jig like a straight edge cutting guide for a circular saw where you take a strip of thin ply (1/4 to 3/8") and laminate a guide strip on it. When you make this jig you leave the bottom strip a little wide and cut off the excess on the first pass. The downside of this jig is that you have to use the same bushing and straight bit every time and they have to be centered every time so this method winds up being more complicated and time consuming.

Here is an article about the circ saw guide so just imagine two of them joined together and on a smaller scale. https://www.woodmagazine.com/woodworking-tips/techniques/sawing-solutions/circular-saw

If I was me, I would build one just like mine. Works great, no problems. Make it what ever size you want.
http://www.routerforums.com/jigs-fixtures/49738-exact-width-dado-jig.html

My jig looks like the one in Mikes last photo, except my jig has a strip along the outside edge on top of what Mike shows in the last picture. These strips guide the base of the router to keep the bit centered, and when making the jig, you trim the edges of the slot using the router bit while the base of the router rides along each of these strips.

In use, the jig needs to always be used with the same router, router base, and bit, but it will cut a dado the exact width of the gap in the jig every time. So you can set the jig by putting the board to be inserted in the dado in the slot of the jig and move the two halves of the jig until it is tight against the board. Then lock the jig setting and remove the board. Place the jig on the board to receive the dado, position the jig where you want the dado and then clamp the jig into place. Set the bit depth and rout down along the left side of the jig and then back on the right side. The dado will be the exact width of the board used to set the gap width in the jig. No collars or bearings are needed. The router base and these upper rails of the jig do the guiding instead.

Charley

The best method for dado cuts using a jig is to use a template guide of KNOWN external diameter and known offset.

e.g If you are in America and use the imperial system, it is easier.
A 3/4" template guide + a half inch bit (1/2") will have offset of 1/8" always.
A 3/4" template guide + a quarter in bit (1/4") will give an offset of 1/4" always.

Template Guide is best suited for Dados because unlike the pattern bit (bearing guided) you can vary the depth of cut just by plunging which is impossible with a pattern bit (bearing guided) unless you buy a pattern bit with a very short cutting edges and a very think template.

BUT you have a BOSCH and I think their simplified optional dust chute for your router will not work with template guides (?).
So for Bosch users I can only say use it would be better to use SHORT pattern (bearing guided) bits.
An annoying thing is that a 1/2" pattern bit (most useful size) with a 1/2" bearing will by physical limitations only come in a 1/4" shank.
FYI: Pattern bits + jigs were designed for through cuts & trims not dados of varying depth.

A suitable pattern bit for dados

Note
1. Short blades cutting edges
2. 1/4" shank if 1/2" bit
3. Require thick templates.

Mortising bits like that cut nice flat bottomed dados. Also useful for installing hinges flush with a door or jamb. I used one of these with a 1/4 inch shaft to slightly deepen the rabbet for a thicker aluminum router plate, and to install hinges on three interior doors that replaced the old, teenager abused doors. You definitely want the half inch shank for dados and grooves.

The illustration is how I set up a pattern to fit the new mounting plate. Fit the 4 pieces around the edge of the plate and clamp them down. I'd add either a card or painter's tape to make the opening slightly oversized (or it will be hard to lift out).

For those with track saws made by Festool, Makita and Triton, you can purchase a router base that runs on a track. This could be used to cut a dado, but is more likely to be useful for simple grooves or rabbets.