Question for you technique gurus...I'm testing TA. Add 5 drops, still green. Add 6th drop and it turns to a medium pink, no green. That would be 60ppm. When I add a 7th drop for 70ppm it goes all the way red. Another drop has no effect on the color. What is the correct way to read this? 60ppm when it turns medium red/pink, 70ppm for pure red or 65ppm in the middle? Is a "middle" reading ever used on the color change tests?