The clunk should touch, or almost touch, the outer wall of the tank when the plane is held with the wings vertical. If you have a Hayes tank and this is not happening then your line is too stiff. If you have a DuBro or Sullivan tank, then either your line is too stiff or (particularly if you have a slant tank) the brass tubes are not bent correctly.
I've never been fully satisfied with regular tanks -- even the metal tanks that find their way into my airplanes end up being clunk tanks. Clunk tanks can and do work very well.