Seriously no lost juice yet. I just fill the rubber cap about half way and slide the 510 end of the carto into it. The juice fills from the bottom up to the top. I typically let it sit for a few minutes and squeeze the bottom a bit to swish juice back and forth. Once most of the juice is absorbed I pull out the carto and put the top of it against the bottom of the old one in the tank and just slide them up through the juice in the tank while pushing the old one out the top. Once above the juice level I pull the old one out and squeeze out the last drop or two in the rubber end piece left over from priming into the top of the tank. Slide the new carto up the rest of the way through the top ring and let it sit for awhile.
I stocked up on flanged cartos for my locking and other tanks. Just gave away my last flange lock tank and handful of cartos the other day because the iBT tanks with flangeless is all I care to use anymore. Fast, easy and just plain the least amount of effort of anything I've tried. I don't even use my tanks with filler ports because I find them to be more effort/time.