I think you are mixing up lead with lap for this question? Lead is the amount a valve will be open to steam when the piston is on dead centre. Ie lead introduces steam to the cylinder before the piston goes over centre, there are a number of reasons for this one being it provides a cushioning effect to the piston. Lead is determined mainly by the setting of the valve gear, in Stephenson's link motion this will be the angle of advance of the eccentric at full gear. Lap on the other hand is the amount the valve overlaps the outside edge of the steam ports. For example if the measurement across the outside edges of your steam ports is 12mm and you make the valve 15mm then it has 1.5mm lap (overlaps the outside edges of the ports by 1.5mm each end when the valve is set mid travel). Lap also needs to be considered when timing the valve gear. To get a real understanding of lap and lead you really need to study valve gear and the implications of valve events and the different types of valve gear available and the idea of variable or constant lead.

Paul.