are you asking about shims under the legs on tables with no levelers? it seems to me that any material that could support the weight and didn't compress and was the correct thickness would be acceptable. I am laying granite tiles 4.5 by 9 under my table and setting them perfectly level, to negate use of shims, my legs don't have levelers. As far as the slate itself goes, using the blud method seems time consuming and overkill to me when you can do just as good a job shimming at each screw. If your frame is level to begin with, how much do you have to shim anyway?