Hi ordinary Bill,
I built mine at about 200mm or 8 inches from the top of the slats and it gives me about 5 1/2 inches of cut depth allowing for the bottom rollers on the X axis. This makes for a relatively stable design without too much flex on the gantry. I'm using 4mm (3/16) thick 100mm (4") square section for the X axis. I wanted to make it higher just in case I had to cut out a part of a tank or box or something but then I realized that all you have to do is remove the slats and position whatever thick thing you have underneath and prop it up to the cutting height.