[QUOTE=Kevin Kehler;1485613]A film/developer combination which needs 20 minutes of time does not exhaust 1/20 of it's power every minute. Rather than being a linear relationship, I would think it is a sliding scale, where 40% of the development occurs in the first 5 minutes, 20% in the next 5 minutes, etc., etc. /QUOTE]

Just to back this up I was looking at a Kodak instruction book some time ago where there were several pictures of a neg developed from 2 mins to,I think, about 12 mins

I was absolutely astounded at how much the neg had been developed at as little as 2 mins which was only about 17% of the full time.

It certainly made me wonder at how much we need be concerned at mistiming by 30 secs or even 1 minute in a 12 min dev time