Determining color change for TA

[aquarium]

For example, say you put in 9 drop and got a red color, then a 10th drop and it got redder, then an 11th drop and the red stayed the same as the 0oth drop. You would not count the 11th drop and your test ended at 10 drops.

But is the reading really 10? Since you got a bit of change at 9, I'd guess that the actual reading is somewhere between 9 and 10, or in the case of 10ppm/drop, somewhere between 90 and 100ppm. Which is why it seems approximate, to me.