Easy way to work that is to have your cfg file in the appl directory look for both and set based on which one it finds.
An example would be:
%if exists (f:/Bentley/Standards/.)
_USTN_SITE = f:/Bentley/Standards/
%endif
%if exists (z:/Bentley/Standards/.)
_USTN_SITE = z:/Bentley/Standards/
%endif
The way this works is that _USTN_SITE is set based on which path is found. This way you can use your different locations, but have only one cfg file.