This is what I did 10 years ago and it is still going strong. How about a turn/jack screw placed on the floor and under the middle of the table tops sag to screw up until flat. Placing a level over the sag to evaluate how more screwing is necessary. I placed a 2' x 4' piece on top of the turn/jackscrew to push upward from under the table from the floor to assist with leveling out the top of the table. I check the level from time to time, especially if I am building a new model. Works without fail. Chic