Good point, Opsec. I've done similar: When opening a bag of rice, sugar, flour, laundry powder, t.p., etc., etc., I write the date of opening in a ledger. We've been seriously prepping for 9 months now, and have developed an awareness as well as a record of exactly what we use, and how much. This not only helps us to plan future purchases, but enables us to work out how to take the best advantage of items we find on sale, (which has also translated into some real savings for us).
We have mainly built up our stores on the basis of buying two replacements for each one item that is used from the pantry. The exception is when I find something on special - e.g: canned tomatoes @ 20ยข ea, (when I grabbed a dozen cases). I then adjust my preps budget accordingly.