You could use a syringe - fill that with the amount of e-liquid that is recommended for your size cartomizer.
That is a sure way for you to know how much e-liquid.
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I just wrap a square of toilet paper around the bottom(threaded end) and place an individual drop down the wall,
let that absorb, spin, do a 2nd, let absorb, spin, do a 3rd(3 points of a triangle) - then flick it like it was an old-time
thermometer. ( I don't know, I could be getting old or it's all the many types a handwork I've done in my lifetime but
I'm thinking if I'm flicking these cartomizers for the next 20 years - I will eventually end up with carpel tunnel.

)
My first carto fill, I was peeking under the toilet paper right from the start, checking to see if the liquid was coming out
the drilled hole in the wall of the cartomizer and showing on the toilet paper yet -LOL-
each cartomizer to follow I dripped drops and flicked a little more often before I peeked.

And I'm only at about
7 cartomizers used so far (early days/carto newbie). LOVE them & all 7 have been worked perfect. First ones were
Smoks but now using ikenvapes nitros(I'm done searching - the nitros will be the ones I use from here on out.)
My current nitro won't die and I'm chomping at the bit to try one of my new flavors.
I read a lot here & watched a couple instruction videos - not just on how to use them but on what the heck to even
purchase to start out with, crossed my fingers and clicked "buy". I must have comprehended fellow ecf members as
all my purchases have been successful. Phew!
I gave syringes a try for filling a carto but didn't care for that process - maybe because the needles are blunt
tipped and I can't insert it between the wall and filler - so to me there was no difference between syringe &
e-liquid bottle. Then I felt that wasn't worth the hassle of taking apart the syringe and washing it & letting it dry
completely since it didn't change/improve the priming process for me.
I don't regret the syringe purchase($2) - they are used all the time to get the last bit of liquid out of my bottles and
to remove the liquid from my tanks when I replace cartomizers or change flavors.