Hi,
Not too much difference between a nickle and a dime when to compare them to each other. When you figure a round ball to a rounded tip, the surface area of contact footprint is a mute point.
I pretty much agree with Chris and let the dia. dictate the radius.
I had the chance to do about ten shafts for Tom Rossman when he was at my pool hall. I asked him if he wanted a nickel or a dime and he reached into his pocket and pulled out both and showed me that they are pretty darn close. He said just give him an even radius and he will be happy.
Rick