Mike

John, when we are setting up for a mortise cut we generally mark the wood and this can be easily seen with most methods of mortising. The Oak Park design uses a 7" square mounting plate with roller bearings on the corners for stability. Rockler recently introduced a sub base plate with screw in pins to accomplish the same job. It's far cheaper but lacks the stability of the 7" plate. There is no reason why you could not install pins in your routers sub base plate to do the same job. As far as fit is concerned, the pieces should fit together well but not need to be forced. Yes, you need to allow room for glue squeeze out. If the parts are too tight and different woods were used you might face wood splitting with temperature and humidity changes.
The "sideways" pivoting mount works well. As you pivot the mounting plate up the bit raises. Once you are at the desired height you lock it in place. I think this is what has you confused: the starting and ending points will vary depending on how thick the wood is and how high the mortise is from the table sliding tray. The higher you set your bit, the closer to the pivot pin you are.

A router bit will leave a half oval end in your mortise which usually is ample room for the glue squeeze out.

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Mike
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