re: Angled dado?

I'll typically do what Bently first proposes, just angle the dado set in the TS and cut. There is the issue of watching the thin area at the point as to not make a weak area.

Bumping up/down a narrow blade as you move over to get the width will leave a rough bottom, but a chisel can take care of squaring it up (a lot of work)
Alternatively you can just do the cut, then using some scraps of the poplar, bevel the edge to match your shelf angle then rip cut it off to make a fill strip to angle the dado cut, glue into the cut and you'll have your flat.