Three bits of data to add to this thread today:
1. A Hyundai Kona's windscreen washer bottle will take exactly four litres of water before it starts to rush up the filler tube.
2. ACDelco Optikleen mixes at 10mL per L of water, so the litre of it I bought will give me 25 full refills in my Kona.
3. While putting the Optikleen away in the garage I found a full, unopened 1L bottle of Bar's Bugs I have zero memory of acquiring, so I now have approximately a lifetime supply of washer fluid.