I suppose the thickness of the MDF would have considerable influence on the appropriate spacing of any screws.I've used 12mm,18mm and 25mm on different machines with only the 25mm used without vacuum hold down.For the kind of use described,would a really thick and substantial slab of MDF be useful?Something like 40mm would be unlikely to move too much,even though it is a real pain to lift around the workshop and 50mm really needs four people to lift a sheet.In either case you would get away with only installing it once and skimming it when say 1/2 inch has been consumed-at which point you glue on some more 12mm and give it a skim when the glue has cured.I suppose the concept I am laboriously describing would be that the fasteners are used to stop lateral movement and the stiffness of a thick slab of MDF resists vertical deflection.