I think the position of the spoilers may have a lot to do with what you're trying to achieve. If you just need to dump some lift then the 20 to 35% back from the leading edge probably does the best job of spoiling the lift. But for directional control Mike's numbers may be better for leverage with less yaw inducing drag.

Here again the full size aircraft roll spoilers all seem to be farther back.

As for the proverse yawing. I would think that would be desireable as long as the drag induce yaw was in harmony with the rolling action.