Opinion is what you'll get. Mine is that it depends (surprised right?) on range, type of shooting, and personal preference. My opinion for the precision rifle matches that you mention is 4-16 power. Clarity is far more important that power though. Budget money for the best glass you can afford, then add a few hundred dollars. I cannot emphasize enough the importance of quality glass. I would much prefer my NF on 6 power to an inferior scope on 10, 12, or 20. I think your thoughts above about the 3-20x50 range are about right. I shot with a 6.5-20x50 mark4 for a while (really liked it), and I now use a NF F1 3.5-15x50. I don't really miss the higher power except when measuring subtension, but I'm glad to have the low end and the extra (IMO) usability of the scope.
Just my thoughts so YMMV, but go for quality over quantity (power).