There is a basic misunderstanding implicit in the question; namely, the assumption that development times given by the manufacturer (or other source) is the "correct" development time for you. Manufacturers in particular tend to extend the recommended development time to ensure that you don't loose the image (by underdevelopment) because of their recommendations.(posted 8313 days ago)Each photographer should experiment to come up with their own correct development time that will be influenced by many factors such as: agitation technique, type of enlargement light source (dichro, diffusion, cold-light, condenser, etc.), amount of solution per roll of film in your tank, etc. How you account for pour-in and pour-out time is just another one of these variables that is specific to your own technique. As mentioned in the responses above, consistency in all these factors is essential in coming up with a development time that is correct for you.
There are techniques for using the Zone System to methodically determine the correct development time for you (other just than looking at the negatives and judging that they look "good"). These are explained in various books, many of which are available in your library or bookstore.