measuring for pushrods. .please heelllllp

First off I bought a pushrod checker. . The 7702 comp cams one. And I have no clue how to use it. . I seen a video on youtube a huy was showing how to use it. But ona ford. Any way he had the same checker. .thing is I dnt even know the length of the checker when all the way closed how do I fing this out..and wats the procedure on measuring gor correct length??? Thanks guys