It wasn't my original reason for recommending the DPD-FAS method of testing -- the idea then (nearly 20 years ago) was just to get people out of pool stores, and away from bogus testing and predatory 'chemical prescriptions'. But the fact is, it is hard to do proper testing of appropriate chlorine levels without it. Once you've gauged your pool exaclty, you can use an OTO drops kit to keep a 'rough-n-ready' eye on it. But there's really not any other way, besides DPD-FAS, to set things up, and keep them on track.

Regarding the "first time I saw", etc -- as far as either I, or Richard, know, I was the first to make practical recommendations based on the CYA/Chlorine relationship. Richard started his quest here, around 2005 I think. And neither O'Brien or Wojitowicz (the Olin chemist) made any practical application or recommendation.

And while I strongly suspect that Jock Hamilton, who founded United Chemicals, understood the relationship, to at least some degree, long before I did, he kept any such understanding in his vest, and used it as a springboard for his sales of bromide based products to create a 'free halogen residual'.