This is needed for custom items, for example if I need a special medication tree times a day, I shoud be able to add it to the default list with a note "3 times a day" and the app shoud calculate the number of pills needed for a given trip length.

Another example would be editing the default number of T-Shirts, I might want to plan changing a Shirt every 2 days, so I should be able to customise this.

Calculating the number of Items needed is a feature missing in all packing apps I tested so far. PackPoint is the only one that does it, it just needs support to customise this feature to be perfect.