Why do you need such accuracy at this range?
That's a question you might want to ask.

Assuming you do......
Try making some known standard solutions.
Say 1.2, 1.4, 1.6, 1.8, 2.0, 2.2 etc over the range of interest.
Then use these to gauge the test kit.

I use a colorimeter for more accurate test.
Color is tougher with your eye.

Regards,
Tom Barr