The Paddleducks engine designed by John Morse is a spool valve, and on this engine the eccentric offset is 3mm, total travel is 6mm, and each spool on the valve is 3mm. The steam ports are also 3mm, and spaced 16mm apart. The outer ends of the spools are 19mm apart, but that is measured before making the valve.
The general design is that at center position both valve ports are closed, and at full stroke both are open, one to steam and the other to exhaust. It's unclear to me that shortening the stroke and thus reducing the exposed port areas offers any advantage.