Path File Names

I am having a problem with the GetOpenFilename function (Win32)
I am getting the full path name from within Blitz3D program but I want to get the relative path. For example:
My Blitz3D program runs on:
"C:\My Programs\My Program"
and I am selecting a file from:
"C:\My Data\"
I am getting:
"C:\My Data\Selected.file"
but I want to get:
"..\..\My Data\Selected.file" (relative to "My Program" folder)
Any ideas?

Maybe this routine ? I haven't tried it:
http://www.blitzbasic.com/codearcs/codearcs.php?code=1187

Thanks bram32!
I didn't test it but it is exactly what I was looking for...