Hi,
I think once the hesitation of making a mistake wears off you'll find you can:
Take the plastic tip and rubber washer off the cart, then partially place the the cart in its rubber shipping cover (like just a 1/4 inch insertion leaving a generious empty space beneath...
now with it set up like this with a reservoir to catch any excess
juice you can just filler up---
any extra juice gets caught in the cap, plus you can squeeze the empty rubber shipping cover and work the juice a bit....
I squeeze the the shipping cap (once the juice is added) then cover the cart mouth with my finger tip--release the pressure on the rubber shipping cap---take my finger off the cart's open mouth, and watch as the extra juice gets sucked down...
Might as well have some fun...eh?
Need a picture?
Anyway, This is my technique, and I suck up the overage from the rubber shipping caps with a pipette and squirt it back in my juice bottles,,,
PERFECT
no need to be fussy
Regards
fo