With geometry, I want to center text body and margin notes together, namely to center textwidth + marginparsep + marginparwidth parts of horizontal elements.
I think one option would be to include margin notes into total body by includemp and center the total body. But that would screw up the current layout of my document and I have to adjust hscale all over again.
So how could I do it?

\dimexprprimitive you used in your solution. – Naitree Oct 02 '14 at 06:19