IDLER ARM QUESTION

In theory
The center of the idler arm ball and the center of the pitman arm ball, should be at the same height relative to the ground plain.In practice, this is hard to measure. But the whole reason for this is to set the centerlink parallel to the ground-plain; and that is much easier to measure. Doing this will minimize bump-steer.
In reality,
that washer makes so little difference, you might as well put it where ever it wants to be for header or oilpan clearance;or where ever it is easiest to install.

When I installed those offset Moog UCA bushings and set the caster to the maximum that they would allow with about -.5* camber, the toe-pattern went a little bonkers. During the rectification procedure for that, I had to slot out the lower idler arm K-frame hole, in conjunction with that washer for height adjustment,to put that arm into a correct working angle. Afterwards I tack-welded that washer in, so that it became the new lower anchor point,and that when it came time to replace that arm, the replacement would hopefully drop right into the correct working relationship.