The best advice I can give you is to meter carefully, look at your notes from previous shots and similar contrast ranges, then decide how the print needs to be made. If your shadows are correctly placed and you know a critical portion of the print will be lifeless, add to your development times. Similarly, if shadows will be good and highlights are above what the film and paper can handle (from experience and careful metering), a reduction in development is needed.

Most of this is subjective. I live in the desert, so one would think that the scenes would be very contrasty. This is not necessarily so. For landscapes, I find myself using expanded development fairly frequently, because there is a rather narrow range of values when there is no sky in the image. The spines on a cactus don't really come alive without a little bump in processing to make them a bit lighter than they actually are. This is not the actual light one would see, but it seems to look better than a standard development would in the print.

The best advice I can give you is to start with about 20% as a point of departure, if you want to add or subtract brightness.This will be enough to see what is happening. If you take careful notes, time will show you the way. Without decent notes, it is more difficult to begin to understand this process. After a while, the notes are more trouble than they are worth. But for learning, notes make things easier to recall and understand after the print is made and studied for a while.

It is interesting to hear how many people will go back to a negative at a later date and make a finer print. It is only experience which allows this to happen.