If you can see it at 40, you're at 30. Or less. Keep going until it is completely gone. Gone as in look away for a few seconds and look again and it's still gone.
There's no shame in pouring the solution back into the mixing bottle, giving it another swirl, and trying again. If you end up between marks, round up.
Some people find it helpful to pour to a certain level and then take a look, add more, look again, etc. That way you don't get the image burned into your eyes. When it disappears, go back one. Meaning it's visible at 40, but not at 30, you use 40 as the CYA value for calculating FC.