There is shrinkage between the mold and the molded part and there can be differences with the 3D printing process - these are two separate issues. I think your best bet is to model the finished part in SolidWorks as you want it to be in the real world and then apply the appropriate scale factor to it in order to make the mold SolidWorks model. That way your dimensions are true and you'll have the separate and appropriate scale factor involved where it belongs.
My experience with 3D printers is not so much one of shrinkage but of "dimensional differences", i.e., some feature edges come out fat and some come out skinny. This is partly a function of the STL file since it is making faceted surfaces out of your smooth model, but it is more a function of the resolution/pixilation of the printing process. I have also found that the resolution of most 3D printers is different in the vertical (Z) direction from the horizontal (X-Y) direction. This is usually stated somewhere in the printer specs. This is not a scaling issue as almost all 3D printers do an accurate job printing to scale, though their precision is affected by the particular situation.
Have you 3D printed a scale test part such as a 25mm hollowed cube, or better yet an octagonal polyhedron? This will tell you a lot about what you can get from the printer in all directions. If you are sending your stuff to a 3D service bureau then they should be a key part of your conversation regarding what to expect from the printer.
- - -Updraft