As we did... 3.5 bottles left over! If we had been better organized and skilled with the stoves it would have been at least 4 and possibly 5 bottles left over. I had a few complaints about the way our trip was organized. I won't mention the organization but in terms of equipement -- we had heavy 4 season tents, superfluous group gear, extra training gear that got little use, 3 partial bags of food left over including some CANNED food that I didn't know about, gorp that nobody could stomach, kayaks that needed repairs, 60-lb 72 litre backpacks that nobody knew how to adjust except me and a couple of people that were all-out jerks.

But enough of that... we were discussing quantities of white gas.
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.