Because I'm an engineer, the answer is always; it depends. The lower knee brace will work most efficiently at a 12/12 slope as any truss member would. The 8/12 solution would tend to bring more of the horizontal force (the hammer!) up towards the rafter/post connection (undesireable).
It all depends on the joinery...the reason for the brace connection at the lower 1/3 of the post is to take out most of the horizontal reaction through the post base rather than the aforementioned rafter/post conn.
In any case you must determine the forces on the joint and design the joinery accordingly or adjust the geometry to force the loads to the desired joint.
I realize this is no answer in specific terms but as I've said I'm an engineer...what did you expect!

P.S. My best guess is to go with a 12/12 brace and make sure the rafter/post conn. has good tension reistance.

Good Luck,

Reid Costley