CYA-I HATE this test LOL It is so hard. Here is what I do...........we call it the bar look!
-full sun light with back to sun and tube held at waist
-mix up water and test stuff making sure it is mixed good
-pour into tube GLANCE at it then look away, glance again. If you can still see the dot, add more mixture. GLANCE, then look away, glance again. Repeat as needed.
If you stare long enough you WILL see the dot LOL I found the "bar method" works for me.
Another person said they close one eye and does it that way. Find what works best for you. Is there anyone else in the house that can look also?
You can do and redo this test with the same mixture over and over if you want to practice. It is better to shoot for the higher value to be on the safe side.
It will not hurt a thing to dose for mustard algae as a last thing before you do the OCLT.
Kim