No answers here, but hopefully some guiding questions.
On router table? Yes
Same bit height? Yes
Bit height? 3/8"
Per your drawing:
Dado is 3/8", which is same as bit height.
Length of tail: 7/16"
Mike, some questions:
1. Are you sure that the dado is 3/8", not 5/16"?
2. Is it possible that one of the pieces is riding the shoulder of the bit? (The Dado might be shallow.)
3. Have you checked the flatness of your table?
4. Have you tried a test joint, using a shallower cut? Try making a quarter-inch deep sliding dovetail.
Your method sounds sound. So, something has to be causing the bit to either cut a shallower dado or cut an extra-long tail.
Edit: I went the wrong way in suggesting 1/2 deep dovetail. Should have gone 1/4 inch.
Last edited by Cassandra; 05-02-2009 at 12:45 PM.