Obtain 18 gauge needles from your pharmacy.

Cut one to the length of a cartridge, and insert pointed side down into the inside wall of any cartridge. As you drop fluid into the cart, the air will be displaced through the tube and your cart will hold waay more fluid than before. This will eliminate the need to constantly top off. It works for any size cartridge.

This idea was inspired by a rude post on another forum. Rudeness does have its place...thankfully not on this forum!