Hi, I have 623X's little brother, the G0700. My process for tuning the slider is a lot like Craig's, and I do use a Master Plate because it gives me a full 10" of flatness to run the dial indicator over.
1. Get the slider even with the cast iron top.
2. Clamp the dial indicator in about the middle of the table.
3. With the bottom four nuts loosened (the ones that go against the lock washers) futz with the two parallel adjustment screws until the slider is parallel.
If anyone is experiencing a lot of movement when they tighten those four bottom nuts perhaps the lock washers are installed on the top side? I've only seen the slider move up/down when I over-tighten the bottom nuts.