Ideally you want the most contact area you can get. I try for about 80-90% minimum contact area between the valve body and the cone/plug but on some cones that is not even possible.
Here we have what looks to be less than 30% contact and the valve had been leaking. Even a thorough cleaning and greasing may have only stopped the leaking for a short while, and it needed to be lap fit. Keep in mind that the main goal here is simply no weeping when open or closed. If 60% contact area in the right spots does it, then you're good to go.
It should be noted that this Spartan Seacock was 32 years old. Show me even one 32 year old ball valve in the marine environment still working and I'd be very surprised. This is why I like Groco's flanged adapter so much. Ball valves just do not last like a true tapered cone seacock does.
In-frequent servicing leads to sea-crud build up in the spot on the cone when it is in the closed position. When you then move the valve it carries this abrasive crud into the valve body thus wearing it down. If you grease annually, and open your valves properly, as in 100% open, not just partially open, you can minimize this wear and prolong the time between lap fitting.