> For a 72 hour kit you want at least 12 liters per person just to survive,

4 liters per person per day? That sounds like a lot. I drink about a liter per day normally. I get some water from food, and use some for cooking, but I doubt it's more than 2 liters a day in total. It'd be nice to have enough for washing but that would be going beyond "just to survive".

[Edit] Incidently, nowadays I buy bottled water from a supermarket. It has a sell-buy date of 12 to 18 months, tastes pretty much like tap water and costs under 20p/l. I tried several different brands and the cheapest stuff tasted most like water. Packs of 4x2l bottles worked out cheapest, and I figured they were more convenient than 1x5l bottle and less likely to go off in the time it takes to drink it when opened.