If you squeeze it out and retain 0.8 ml Kate, wouldn't it be a good guess that the cartridge does indeed hold 1 ml? I think that however hard we squeeze that we'll never get out éverything, the full 100%?
Anyhow, for what it's worth, I do always use 1 ml to fill my carts (of course empty carts; less if they are being re-used without being washed and dried, because then there is still a bit in there from the last time it was filled). I always use a syringe when really filling up a new cart though, and I do believe using that lets you get a bit more in then any other method. I'm just also remembering though: how much goes in also depends on how loose or tight the wool in the cart is... using another brand might indeed make the difference between the 0.8 of Kate and my 1 ml!
Best is to just look while you are filling; as soon as you see the top getting wet, stop putting in more liquid, let it settle; and then perhaps add one or two drops more on top, if that wetness isn't intense yet (but never so much that there is 'loose' liquid on top, then it is indeed too much if you used a syringe - with other methods this access might still soak in given some time, showing the cart wasn't completely full yet).