The dipstick indicates the amount of oil in the pan. After the cooler is installed, fill the crankcase to the full mark, install a new filter, and start the engine. Let it run for a minute or so, then turn it off. Go have a cup of coffee. After you are done with the coffee, check the oil level on the dipstick, and add to the full mark again, keeping track of how much oil you put in (duh). That amount plus the original fill amount will be your new capacity.
Sorry for the oversimplification, but the next guy reading this thread may not be as mechanically inclined.