Testing iron with aluminum
The first thing I'd do is turn your aluminum ruler over to see if you still have the gap. I measure with three aluminum levels, and I measure off each side to ensure that they are flat. If you get convex on one side and concave on the other, then it's not really a straight edge, is it?
I only mention this because aluminum is not particularly well known for its thermal stability. It can warp fairly easily.
As for the adjustments I make, I keep it pretty low tech. I have "beams" (for lack of a better term) that I made from plies of MDF and baltic birch that I clamp across the table and span the extension wings. I clamp them rather loosely at first to the extension wings (just enough to help support the weight), loosen the bolts holding the wings to the table, place clamps on the joint between the extension wing and table, then tighten down the clamps on the "beams".
This, in theory, is supposed to bring the top of the extension wing perfectly in line with the top of the table while you re-tighten the bolts. In practice, you have to hold your mouth just right while softly humming "How Great Thou Art" to get them to line up the way you want them. The only way to absolutely positively guarantee that you're going to get them adjusted to optimal flatness is for you to be more stubborn about it than the saw is.
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