My understanding from this is that sectional densities are valid only within the same caliber (and within a certain velocity). Use them that way and they'll be helpful: try to mix and match calibers, velocities, and bullet weights and you'll get problems. Is this close enough for the average Joe?

Here's a link for the Linebaugh penetration tests.

http://www.handloads.com/misc/Linebaugh.Penetration.Tests.asp?Order=5