How is stem length (reach) measured? How is the length of a stem measured? Where does the measurement start and finish?
It sounds as though you are talking about what Sheldon Brown term "reach", which is 


  normally measured center-to-center along the direction of the extension


So this does not take into account the angle of the stem, which could reduce the length forward that the handlebars extend by a small amount if they are on a large angle up or down.