Sorry to hit this thread so late.
This sub-forum is usually pretty dead and its one of the last ones I hit.
There's multiple ways to prime a carto. No one way is the best. Whichever one you find works for you is the one you should settle on.
You can inject the filler with a needle/syringe, condom fill, reverse condom fill with flanged cartos - The Ocelot's preferred method, fiddling with your thingy (OCD's method), and Baditude's method. Of course I've had the most success with my own and made a blog describing it.
I drip from the top of the carto, before putting in into the tank. I use a wood tooth pick in the center air hole to keep juice out (this avoids having to blow out the carto later). I don't count drops, but you can. Different size dropper tips make different size drops, resulting in a potential huge difference in the volume of liquid used. I use my experience to observe how saturated the polyfill is. A standard length carto will hold 1 ml of juice; an XL length carto will hold 1.5 ml of juice fully primed.
I drop in a few drops, then shake the carto down like an old school mercury thermometer. Add a few more drops, shake it down again. I repeat these two steps until the filler will hold no more juice. It will look like a wet slushy drink; glistening with juice with no pooling on top.
I remove the tooth pick, add a tank tool, and insert the carto half way into the fully assembled tank. I fill the tank through the top end cap 80% full. Push the carto up all the way. Remove the tank tool and replace it with the drip tip. Finished. The entire process takes me 5 - 7 minutes. I may accidentally waste a couple of drops of juice when adding juice to the tank if I'm sloppy. No muss, no fuss.
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I use both flanged and unflanged cartomizers. The flanged cartos are only needed in some Smoktech tanks. I've found no need for a flanged carto in an IBTanked. Once the o-rings absorb some juice and swell and then dry out the tank holds securely on the carto.
I use two hole prepunched cartos. Most of my e-liquids are a 50/50 pg/vg ratio, but when I do DIY I use mostly Vg for more vapor. I may need to do more primer puffs while vaping with the thicker e-liquids.
It is normal for the polyfill to look dry after vaping for a period of time. Not only does juice get pulled from the tank with each puff, juice is pulled down in the cartomizer, too. The punched holes will keep the lower portion of the carto nice and wet (where the coil is). I've not found it necessary to add juice to the carto during tank refills, but no harm will be done if you decide to.