You are pretty close. The problem is if you run the 10ml test, you cannot tell if it's actually .5, or less. If you see NO pink tint at all, it's 0ppm. But if there's a hint of pink and one drop of reagent clears it, it could be anything from just over 0 to .5, but no more.

A lot of us have found it to be very difficult to get that last bit out--and swim anyway. It can mean something--I had a very tiny but persistent bit of algae inside my drop-in steps that seemed to not be reachable even when FC was 15! This year I'm trying something else that seems to work.

Remember, though, you CAN do the test with 25ml. Then each drop represents .2ppm, not .5ppm, so you can get a better feel if it's only 0 to .2ppm-if it's definitely less than .5 (.2 or .4ppm) then don't worry about it.