Running low on pet food - notification

[WitchDevelops]

Another friend of mine says she'd love to get a notification/reminder to order pet food a week before she runs out.

This could be implemented as a tab, where the user would put info of how much food they have (in mass units, kg/g), how much the pet eats per day, and calculate it.
Or in case of wet food we change the setup for cans and cans per day. (but they come in different sizes)

problem: this could work well for a single pet household and only when you know exaclty how much the pet eats. For multiple pets households and when you don't measure the amount of feed it won't work well. But perhaps 1 week as the error margin (so that the notification is not sent too late, but on time, when there is still some feed left) could work?

thoughts? ideas?