Keeping packaging material and photographs in sync with inventory

Hello!

At my Purchases spreadsheet https://docs.google.com/spreadsheet/...EtwX0NrQk9pR2c - can anybody recommend a way of calculating when it's time to get more packaging material and photographs - and how to best model this in my spreadsheet?

Thank y'all so much!

Mikkel

P.S. Anyone can edit this document.

Also, my spreadsheets are originally Excel. In Google Docs, click File > Download as > Excel.