The amarillo at flameout seems slightly heavy-handed for an oatmeal brown, although if you're a hop fan, it'll probably still be tasty. I might up the chocolate to .25#, as it should help balance the hop flavor a bit.
Halving them both would probably be a bit more brownish, I'd say. And the varieties aren't exactly traditional for an English brown, but since you didn't state that's what you're after, they look fine. Magnum plays well with anything, and Amarillo and Cascade are wonderful together.