Relative include path for a vpwlf source

I'm runing a spectre simulation via ADE-L in 6.1.5

I want to use a VPWLF signal source, and I would like to point to the file with a directory path which is relative to my cadence startup directory (i.e. my design workspace). All the documentation I have read talks about directory paths which are relative to the netlist directory, but this does not help, because there is no fixed relationship between our design workspaces and our simulation directories.

Tony