Sorry about the format but it'll only take a couple of mins to type in. I've left alternate cells empty because I like to put in coloured lines between boxes.
At Cell B6 type in Adults in group
At Cell D6 type in Food Preps Stored in Days
At Cell F6 type in Additional Adults
At Cell H6 type in Mouths to Feed
At Cell J6 type in Food Now Lasts Days
Now the fun bit.
In Cell B8 put in the number of Adults your actually prepping for
In Cell D8 put in the number of days food you have stored.
In Cell F8 put in the number of extra adults who turn up with no food of their own.
In Cell H8 enter this formula =SUM(B8+F8)
In Cell J8 enter this un =SUM(B8*D8/H8)
As you fill in the boxes the Mouths to Feed and Food Now Lasts sections complete themselves.
That's it. Have fun (or sleepless nights)