My perpetual Inventory sheet

To save a copy: Go to the following link and then select File > Download as > and select format choice.
http://spreadsheets.google.com/ccc?key=0Ai3zK3S0SNBWcGxzTUl2bWlsU2s0dkpVNnI3bzB4RGc&hl=en

There are different methods to determine how much you should store per person.
The best answer is Your own personal experience based on how much you eat!!
but here is one calculator the Church used less than a decade ago.
http://lds.about.com/library/bl/faq/blcalculator.htm