I'm not sure if this is what your supposed to do.. but its what I did..and it seems to have worked pretty good.. I GUESS I am lucky the warp is uniform across the head slate ..and its a bow, down to up from the head seam, and is mainly in and area ABOUT 3/4 from head seam to head, where it bows for About 9"..What I did was put two shims on each side of the nine inches .. inserted the slate screws on the head . then slowly pulled the seam slate screws down to flush the seam..this has improved the level. I'm going to let it sit for a few days.. recheck ..and slide a few more shims under it.. Again ,,read the first line of this post. any positive or negative feedback of this procedure, welcome, What would you do?