Depends upon the type of grain focuser. The ones which have a reference line are meant to be operated using the parallax method, not "when the grain looks sharp." I know the old Bausch and Lomb worked that way. I think the Scoponet does (have not used my Scoponet in quite some time and I just don't remember.)
Positioned as folks have noted above, you focus until the grain looks sharp, whilst you are looking through the center of the focus finder. Then you tilt the head back and forth slightly. If the grain moves in relation to the reference line, you are not quite in focus. Refine focus until there is no displacement when the parallax changes.