i can't help you with the "correct" number for the stem. however, nothing irritates me more than guys that use, or have installed, a stem that is WAY too long !
that includes ALL types of wheels, be they steel without caps or covers, trim rings, mags of all kinds, wheel covers, etc.
to me, nothing looks worse than a valve stem sticking out in the breeze. it sticks out like a sore thumb.
i use a stem that is just enough length to be usable with whatever wheel setup is being used.
remember now, this is just my opinion. your mileage will vary.