I haven't read this anyplace but it helped me determine that my table is square a few months ago and it might help you square up your table.
I have an old 3' X 5' mirror (no frame) from a 20 year old dresser. I checked it for square and it is about as perfect as one could want. I laid the mirror on the bed of the table along the edge of the rail and checked to see if the table was square. I figured that I could loosen and tighten the bed bolts as needed if it was out of square.
As it turned out the table was already square to within less that 1/32" on my Gold Crown III at the worst place. I figure that over a 9' distance that is pretty darn square given all of the components.
I have now moved this table between four different houses and it comes out square everytime. For all the discussions that people have about who makes a good table there is something to be said for the engineering quality of Brunswick (can't help a small accolade for their engineers).
Anyway perhaps something like this will help. Seems to me that the bed bolts can be moved as needed. If not, I would not have a problem reaming the holes in the slate and then packing the loose side with Bondo or hydraulic cement as needed. Perhaps someone has a better idea.
Another trick that would work to get a perfect square is the old carpenter's trick. The two diagonal measuremts should be identical. I would use a piece of metal corner molding or tile molding as a measuring stick. Costs about $5.00 at Home Depot and could be cut to run from corner pocket to corner pocket. Scoring the metal would give exact distance to the corner pocket shelf.
Then too you could place a note in the table mechanics forum. They probably have even better ways.
