HI ,
first you have to create parameters
volume
mass with realnumber variable.(by using parameter command)

then u have to give relation

volume=mp_volume("")
mass=mp_mass("")

here mp_mass("") and mp_volume("") bothe are calculated by system.
now we are assigning the value to your variable(volume and mass).

this all should be done at model.

now you have to retrive this to your drawing.
make a note &mass and &volume.

now you will get mass and volume which is calculated by model.

takecare

with love
felix