Since FreeCAD 0.17 one can do this with "Expressions". Click the Formula button in entry objects and you can make a formula that contains references like "someotherobject.propertyname".

For the central driving paramterss I recommend using a spreadsheet, as others have mentioned. I usually call that "inputs". And then have another one called "outputs" if I need to calculate something based on geometry, like total material/weights etc.