I agree with CBXBob! I'd maybe use a tiny amount of the LME with the steeping grains, and add the rest at flameout.
However, Magnum and Centennial are both pretty high AA% hops. For the Centennial I might do 1 oz @ 15' and 1 oz @ 5, or maybe 1 oz @ 10' and 1 oz @ 5', depending on the AA%...