David,

While Randy (Worker) is correct about small changes making a difference, in practice, the process taken as a whole, is incredibly forgiving. You could miss the time by 10-20% or temp by several degrees and still get very usable negatives. That amount of time or temp adjustment is used regularly to refine how a negative prints.

If the negative ends up a bit under or overdeveloped, the paper grade (contrast) is simply adjusted to "fix" the problem. Adjusting contrast at the paper is normal and very often used even with "properly" developed negatives.

This is not to suggest that you should be sloppy about your processes, just that it isn't worth worrying much about in the very beginning.