Awhile ago, someone made a topic about a rough calculation of silver when there is silver showing on the rolls. I wish I could find that post!
I remember that topic. The consensus though is that it really don't matter. If there is 1 silver showing it can sometimes be the only one in the box. Happend to me several times. Many times I've found as many as 10-20 silver in a box that did not show any silver.
I do have to say that when I open a box and I see silver I would like to imagine that I will score at least 5 keepers for every silver showing.
Mojo