Yep, I'm with Marian.
You have to make sure enough light gets in to illuminate the suspended particles, which is what obscures the dot. That's why you hold it at the top and use bright lighting. Too much downward light and the dot gets illuminated too much, hence why you shade the test tube with your body. The light is being reflected from the ground and surrounding objects, lighting up the suspended particles, and obscuring the dot.
I agree with Triptyx too - don't sweat over plus/minus 10. Pick your number once a month and go with it. A little more chlorine than target doesn't hurt anything or anyone. So for you at 40, and 3 minimum, Marian will dial you in with an effective evening target, or morning, or whatever time of day you want to do it. She's smart, especially after eating chocolate cookies.