I don't know about creating a custom unit of measure to scale the model,
but you really need Tool Design Option (TDO)
This will (among other things) allow you to have a 'real' shrink factor
feature that scales about the barycenter or origin, and allows you to
family table the x,y,and z shrink factors. It also scales any imported
surface features as well. For our uses, the family table capability is
a must as we have one part model with all instances included. TDO will
also allow you to create silhouette edges, 'skirt surfaces' (fixed twist
sweeps) and allow for copy-geom import. At the assembly level, TDO will
allow you to create die stacks, split lines, parting surfaces,
shut-offs, etc...
The 'scale' function normally included is crap. It one-time scales all
of the linear dims by the factor and is not family-table friendly. Any
non-dimensioned features (like imported surfaces) so not get scaled.
Christopher F. Gosnell
FPD Company
124 Hidden Valley Road
McMurray, PA 15317