I would order pushrods that set your tightest one at .020-.030 preload (do some checking on the perfect dimention you should have) then some will be more than that (not the end of the world) but none will be less. you can check your ruler on a pane of glass & with a small square pane of glass you might even be able to use it as a flat check on 4 stems at a time. when checking preload I put a pair of lobes on base circle then ink the side of my small steel scale ruler & set it flat on the head valve cover mounting rails & ink a spot of it with a sharpie then slide it over fast till it contacts the side of the pushrod & transfers an ink mark then I repeat on the other side for the other pushrod then I undo the rocker arm assys till the pushrods JUST stop moveing up vertical & repeat the inking then remove the rocker arm assys & remove the pushrod inked pair & measure the distance between the two ink marks for my exact preload.