In the case of Rodinal, the point is for all practical purposes moot. 1+50 or 1+49 makes very little difference in the scheme of things. The effect of a 10 ml. difference in the amount of water is easily swamped by other factors like temperature and agitation, and even when you start and stop the timer. Don't sweat it so much. Work out your personal development times for the results you want, and be consistent with your practices. That will serve you far better than worrying about a few extra or fewer ml. of water. There are only a couple of films that I like with Rodinal, so I don't use it much. But for those couple of films, Foma 100 and what's left of my APX 100, the stuff works great. For the record, I use 1+49 or the strict chemists interpretation of 1:50.