>A lot of people tend to approximate p=1. This is fine >if you're using say the 60mm micro-nikkor Rather late but an important detail. The 60mm is close to p=1 at infinity but is strongly asymmetrical at 1:1 magnification, making the "P" factor significant (up to about 1 stop faster than p=1) in close up work.
Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.