On many vehicles the offsets are stamped on the inside of the wheel. I do not know if this is the case with maserati.
It's a pain to remove a wheel but if you don't get a response from one of the wizards here soon maybe removing the wheel will help by giving the info you need.
As far as determining additional offset, lightly tape a piece of cardboard down the body over the wheel well area and then measure the space from the wheel body line as depicted by the cardboard to the tire. That will tell you how much more you will need.