Proper usage of CCACHE_BASEDIR?

Hi ccache folks,

I am trying to understand the proper usage of the CCACHE_BASEDIR environment variable.

I have several copies of my product's build tree in various directories that are not direct siblings.  For example:
    /local/mnt/workspace/johndoe/Perforce/main/topdir
    /local/mnt/workspace/johndoe/Perforce/users/feature1/topdir

If I want to share the ccache between these products, how should I set CCACHE_BASEDIR?  Options:
1) One constant setting, i.e.
    export CCACHE_BASEDIR=/local/mnt/workspace/johndoe/Perforce

...or...

2) Change the env variable depending on which tree I'm currently building.
e.g. if I'm building .../main/topdir:
    export CCACHE_BASEDIR=/local/mnt/workspace/johndoe/Perforce/main/topdir
of if I'm building .../users/feature1/topdir:
    export CCACHE_BASEDIR=/local/mnt/workspace/johndoe/Perforce/users/feature1/topdir

Thank you for helping me clarify this!
Jason