A question regarding exposures and developing, which while stemming from the Zone System, is not meant to mean that I am using that system. I recently borrowed several books from the library and have done some research on-line about negative development; a common suggestion is to overexpose to allow for shadow detail and then under-develop to save highlights. This should increase contrast and allow better prints; I also realize that all shots on a role of film would have to be exposed/developed the same amounts. My question is how to calculate these types of scenarios.
I am using a Minolta Spotmeter (digital) so I am able to have very accurate measurements and can have 1/3 or 1/2 steps in ISO (i.e., I usually expose Velvia 50 at 40 as I like the colour and highlights better but I don't develop E-6 myself). I like using PanF, which is a 50 ISO, and then exposing at 25 ISO so there is an over-exposure already; however, looking at the development times (1:1 Perceptol), there is a 4:30 difference between 25 and 50 (I have been following recommended development times so far). HP5+ has a 3 minute difference in ID-11 between 400 and 800 ISO.
So, how does one decide how long to leave the roll in the developer?
I understand that Ansel Adams learned to observe the development of the negative by eye and then dumped it into the stop bath when the density was "just right." Of course, he was developing large negs in a tray...