I recently lowered my front end. The LCA is much more level that it was at stock ride height.

What is the ideal position from a geometry standpoint (line drawn from frame pivot to BJ pivot) at ride height?

As close to parallel to the ground as possible, in my experience if it is perfectly parallel the car will be on the low side, so I usually lower my car about 1 1/4" in the front and that takes care of it.

If it is parallel during ride height, what happens to the RC when the car dives? I also found my car to be too low when the arms are exactly parallel to the ground.

Roll Center drops when the car dives, but lateral scrub is minimal in both directions when the lower arm is parallel.

Just FYI... parallel means the center of the lower ball joint (outer pivot point) even with the inner pivot bolts-- so if you draw an imaginary line it should intersect the center of each lower ball joint and the LCA inner bolts.