I'll give you the ideal way to top up your carts
Keep a paperclip with your bottle, straighten out the long leg.
Poke the paperclip in the small hole, pushing the cart to the top, so 1/8" is sticking out
Hold the cart so your fingernail traps the inner cart right there
Now take the paperclip, and fluff up the wool a bit, so it's even with the top of the cart again
Then slide the paperclip down the side of the wool, to the bottom of the cart, and lean it over slightly. This creates a small air gap to the bottom, so you don't get air bubbles in there.
Now drip in the cart - onto the wool - until it's full. You can tell it's full by leaning the paperclip over a tiny bit more, and you'll see the liquid itself will be 1/2 to 3/4 the way to the top of the inner cartridge. You drip directly on the wool, so the air in the cart can come up the air gap you made with the paperclip.
If you get the liquid all the way to the top, it's too full.
( I actually overfill them on purpose when topping up in dark places, because it's easier to see, and just suck a drop or two back out with the tip of the dropper)
If you need to re-fluff a bit now, you can. Or use your finger to push loose fibers down. The wool should be even, or very near the top of the inner cart.
Now use the paperclip to push the inner cart back down. Don't force it, just until it stops.
All done. I tried to be very detailed, that's why the explanation is long.
In real life, this entire process takes SECONDS, it's very quick and easy. And it fills them perfect every single time
