This will be very hard to explain with just words, a picture is worth a
thousand of my words. And please, someone who has an idea of what I am
saying, please let me know if I am being clear or just confusing the OP with
Cut a 6 inch test piece of CM and both ends are chopped at exactly 90
degrees to face of CM. This is your square miter test piece.
Now, as you have been doing, make a 45 degree compound miter cut on another
test piece, say 8 inches long. We will call this your miter test piece.
This would be as if you were going to make the inside corner a miter cut,
(if I can explain it) the miter goes from the face of the molding out to the
back of the molding, and the back has a longer length than the face.
Let us call the miter cut from the face to the back of the CM the 'miter
waste'. Take a pencil, or some sort of marker, and lightly darken the miter
waste, and only the miter waste. The point(s) where the miter waste meets
the face, make a real dark curved line. Take your time and be accurate.
Now on a flat surface, using some scrap wood as help, lay the mitered piece
on the table at approximate angle it will be installed at (the top higher
than the bottom of the CM.)
Now hold the square piece perpendicular to the miter piece, but at the
approximate angle it will be installed. (Normally this would have the part
of the CM furthest away from your body angled to your left and the closest
part, or bottom of the CM, angled to your right.) Now move the entire
square piece to the right along the miter piece until the square piece
reaches the edge of the face and miter waste. Keep the installation angle
as best as you can.
The contour of the square edge piece should match the darkened line on the
If it doesn't, you have a problem with your compound miter cutting. Post
back and I will try again to explain the process using standard Quarter
If you see that it matches the darkened line on the mitered piece, then you
now know what has to be removed. You want to remove the entire miter waste
along that darkened line on the miter test piece. Aside: you will find
the match up works best with both pieces at the correct installation angle.
Gosh I hope this helps.