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?