I wouldn't even bother with the depth gauges. I have a 4212 and the just aren't that accurate.
Your best method is to use your workpieces as the gauge.
Use a marking gauge to scribe a line on the workpiece at the depth you want. For a 1/2 blind, this would be about 3/4 the thickness. You can eyeball this or measure, both are equally valid.
With your workpiece installed in the jig, you can set the router into place and raise or lower the bit (a fixed base router works better for this task than a plunge base but both work) until the edge of the bit just catches in the bottom of the marking gauge groove.
Depth set. Now route a test piece. Adjustments may be necessary but this is a pretty quick way to set depths.
One key thing about the jigs, they do assume all your workpieces are equal thickness. At least all the sides are the same thickness and all the faces are the same thickness (but a side doesn't have to be the same as a face). Oh, and another thing about the 4212, if your workpieces are narrow, less than 1/3 the width of the hold-down fence, I suggest you add a second dummy piece at the far end to keep things square. This also goes for setting the template height and depth. It is pretty easy to skew the template on this (and most similar) jigs.
Got all that? I don't have time tonight but if it would help I can try and take some pictures later this week.
Last edited by rwyoung; 11-16-2009 at 01:42 PM.