The OTO test is hard to match the color as I find the shades get pretty subtle above 2. Ben's test kit has the OTO test but also has an FAS-DPD test which is the test Janet talks about. That one is much easier to be accurate with as you add drops of a reagent until the color of the solution changes so it is easy to nail down an exact measurement.

Ben sells the FAS-DPD test as a separate item on the poolsulutions page if you don't want the buy the whole kit.

Peter