Where are you located pete?
In all honesty, as soc suggested, get it matched. If you goto the right paint supplier, they will be able to do a formula in your chosen brand for you, and keep it on file, so that should you want some more later on, they can simply pull it up on the computer and mix it for you.
I normally send in the new door trims and have it matched to them, as they are a colour dominant item. Especially once you fit them to the door, it becomes an edge to edge colour, and any variation will stand out like dogs goolies.
Jas