Your countertop needs to be scribed to the wall. Balance the sides up and use something to hold your pencil out to the widest gap. Now mark all along the top edge with that spacer. You can use a plane on the back of the top edge; there is about 3/8" there to work with. It would be a good idea to have the two sides clamped togather before scribing to the wall; just in case the corner is not exactly square, which they seldom are.
Your work is looking nice, keep up the good work!