The answer is a simple one - you use the one head size that your barrel prefers. Most high end pistols will come with a test group from the factory that show what head size your pistol prefers, I generally go with this and haven't had any problems. Most of the time I just used 4.49's because I have a millon of them lying around and make sure when I get a new pistol it has been tested using this head size. The jsb green pistol pellets and R10's in this head size are about as good as it gets :-)
Lee