Rules of thumb can be used. I still think a XX oz./person/day rule is as good as any. If one uses 2 oz/day for one person to 1.75 oz/person/day for a group of 4-5 and 1.5 oz/person/day for larger groups is a good a starting point as any. Then factor in personal habits of whether people have a late coffee/tea, whether they have a hot or cold breakfast, the season (cold weather camping may require snow melting), use of boiling to sterilize water, what type of dish washing is done and whether the group chooses to abstain on cooking on the last day.
You could calculate a similar number when using other types of fuel like alcohol or cannister stoves.
Good points all.
In terms of personal habits, I should mention I usually do the dishes with cold water. Not always (ever try to clean up chili with cold water?), but if I can I'll save the fuel and use cold water.
My 1.5 fl oz/day for one and 2.25 fl oz/day for two is again more of a starting point, a starting point that needs to be adjusted for conditions, style, stove, etc. To me, my numbers are conservative, but YMMV. Still, I figure it's better to have some idea of what you'll need. There's not much out there as I search the web in terms of easily used fuel planning numbers.
HJ