I am trying to derive a part in an assembly from an existing part so that when the first part's geometry is altered the second part will update accordingly. I have tried to use the derive tool but I only need the bottom geometry of the part to derive a base plate part from and I see no way to select just that face when using the derive tool. Any suggestions?
Look into "Copy Object", which will let you pick a specific face instead of deriving an entire solid or part.
Or, in the Derived Part dialog you can select Body as Work Surface. This brings the derived part in as surfaces, which you can use to define your new part, then turn off the visibility.
This sounds as if it might also have been a good candidate for multi-body solids.