router jig needed; angled dado for deck post-railing

Suggestions greatly appreciated on a jif to make an angled(35.5deg downward from the horizontal and reverse on the lower posts) dado in the posts that attach to the stair steps. I've already cut two right triangles to the angle needed but I can't figure out what I need to do as it seems that the edge of the router wants to get in the way. The dado will be on the minimum of 1/4" deep with the profile of a 2x4. My desire is for it to be as accurate a dado as has been achieved on the rest of the posts. Thanks.
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