This is one of those times when a rule of thumb comes in handy. The one I'm thinking of is the "1/6 follow" rule that Bob Jewett mentions occasionally: a rolling CB will follow a straight-hit OB about 1/6 of the distance the OB would travel without hitting a rail.
So in this case, if you hit the 13 ball at just pocket speed it will travel 5 diamonds and the rolling CB will travel 5/6 of a diamond. If you hit the 13 a little harder to be sure it gets to the pocket, the CB will travel a little farther.
pj
chgo
I will check your mathing when ini get home.