Keep practicing with that standard!
For me, I have to stand on the patio in full daylight, but turned so there's no sunlight reflecting off the surface of the test sample. I hold the view tube with thumb and forefinger only so I block as little light as possible. I keep the tube down about my belly button. When all those conditions are met, I consistently get 50 with the standard solution. Which means if I do exactly the same thing with a sample of my pool water, my test results will be good.
Take a reading, pour the sample back in the mixing bottle, give it a shake, and try again. Eventually you'll find the right combination for you. I find bright light, and lots of it reflecting off the white concrete, is the key for me.