howto calculate mkiso created iso size from pathspecs?

[Daniel Johnson <teknotus@gmail.com>]

It isn't clear to me from mkisofs(8) how much overhead the iso
filesystem has.  I don't know how much bigger it will be than the sum
of the filesizes getting added to the iso.  I assume options like
rockridge, and joliet will make it slightly bigger.  Exact numbers
would be nice, but I can live with something like a range from "at
least" to "at most".  Also is there a way to get mkisofs to predict
the final filesize without actually writing it?  This would be really
helpful for writing a script to break a large directory into sets of
pathspecs that will fit on a single disk.

If I missed something in the huge manual please forgive me.