Modifying base part dimenions

I have an assembly, with a part inserted which is actually a part, with an inserted part (base part).  I can't seem to access the sketch dimensions in the base part from the assembly.

Here's the "structure":

MasterAssembly.sldasm

  DerivedPart.sldprt

   BasePart.sldprt

I've tried BasePart@DerivedPart-1, DerivedPart-1@BasePart, DerivedPart-1/BasePart, BasePart/DerivedPart-1 and so far, SolidWorks throws the following error:

"GetParentComponentByNameTree Error: Component not found for name '<insert attempts from above here>' in component path"

Can anyone tell me how to access sketch/feature dimensions in the BasePart?  This is my first time working with a part made by inserting another part - I'm stumped.

Thanks