In my experience, cartridge filling holds onto some eliquid no matter what you do. I did use a straw mod when I used carts which simply put is cutting a coffee stirrer or very tiny straw off the length of your cartridge and inserting it alongside the filling material. I think it helped use the eliquid a bit more efficiently. There are lots of other mods you can do along the same line such as a spring mod...tried that and liked it even better than the straw mod. Do a search of ECF and you will find a few methods...some have youtube videos for us visual learners.
You have other options also such as cartomizers. These are inexpensive and disposable and many, including myself, like them. Some clean them a few times to further minimize the cost. Most vendors sell them.
Others with more experience will be along to give further advice soon.
Good luck finding what works best for you. Don't let it get too complicated unless you like that

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Cartomizers will waste some juice also as they will stop functioning properly..usually just after I fill it

. However, they do hold more eliquid to start with so they need "topping off" less often that carts therefore less fuss and muss.