Might it not be simpler to remember this: (OD - BD) / 2 = Distance of offset. OD = template guide Outside Diameter and BD= Bit Diameter. Subtract bit diameter from the bushing's outside diameter and divide by 2, that's how far the bit will be positioned from the template.
I used an Excel spreadsheet and while I do the math a little different (mechanical design and drafting experence) the results should be the same. It is just faster to look at a chart for me.
I'm not familiar with the Oak Park bushings, but for some unexplicable reason, most of the ones I've seen gain in height proportinate to their diameter. I've had 3/4" OD bushings that were 9/16" long, which forfeits a lot of depth of cut and worse, whatever material you use for a template has to be at least as thick as the bushing is tall to keep the bushing from bottoming out on the workpiece. I can see no reason for this, so I addressed it by cutting all of my bushings down to .20" height. This allows me to use 1/4" baltic birch plywood or masonite for my templates.
I have Porter Cable's set which is really for their jigs and fixtures and they are longer. The ones a lot of people have are made more specialised and have some like a .234 barrel and are sold a lot of places. I should point out that these are mostly PC standard so they do not automaticly fit all routers. They are of course for 1/4" material patterns.
Also remember that bushings are strictly for defining the edge of the cut, they do not control depth and should not be allowed to touch the workpeice. The router base should ride on top of the template in freehand routing.