Interesting...
Don't forget that Richard never used the algae-argument (which is the main driver for our CYA/FC ratio), he only ever argued with sanitation (even though we all know that getting algae under control is vital to ensure sanitation by FC not being consumed by algae and therefore remaining available to kill pathogens - but I guess that argument is less important in professional environments where you can enforce more stringent monitoring of FC levels).
I guess, the important information is on slide 20 of the presentation that gregsfc linked in his first post - here is the link again, so you don't have to scroll back:
https://www.cmahc.org/documents/CMA..._on_Stabilizer_Use._WAHC_2017-10-16_FINAL.pdf
The current MAHC limits allow 90ppm CYA / 2ppm FC (i.e. CYA/FC= 45), equivalent to about 0.009ppm of HOCl.
By going to CYA/FC = 20, equivalent to about 0.02ppm of HOCl, as per Richard's initial proposal, the risk for infections with Giardia would get reduced by a factor of 1.8.
Reading the infection risk from the graph shown on slide 17, the risk for infection with Giardia at the compromise they seem to be agreeing on (CYA/FC = 30, equivalent to about 0.013ppm of HOCl) gets reduced by a factor of about 1.4 - looks like they found their compromise right in the middle.
Infection risk for giardia seems to have been the most fruitful driver in these negotiations, as stated on slide 19: "0.12% risk from
Giardia with current MAHC limits (2ppm FC, 90ppm CYA) gained the attention of the ad hoc committee".
Richards initial proposal would have reduced that risk to 0.06%, with the compromise the risk will only get reduced to 0.09% (which is below 0.1% - maybe that was the magic number?).
I suspect the real magic number was FC 3ppm. That's what the industry currently often states as the max FC level - many manuals for pool equipment specify that as the upper limit, the "ideal range" for guess strips is usually 1ppm - 3ppm. With the ratio of 30 for CYA/FC and cutting off CYA at 90ppm, they found a "scientific" justification to just stick to that range, the supplies industry basically doesn't have to change anything - apart from some fine print that FC 1ppm is only safe for CYA<=30ppm, FC 2ppm is only safe for CYA<=60ppm, and to make CYA 90ppm safe, FC 3ppm is required.
Richards initial proposal of demanding FC 4ppm at CYA 80ppm, would have meant that all the manuals and guess strip labels would have required an update - heaven forbid!
But the important step is that FC and CYA will no longer be regulated independently from each other, but to be maintained following a ratio. This finally acknowledges the role of HOCl as the actual sanitizing species in a chlorinated pool.
What is the impact of reducing the max allowed FC from 10ppm to 5ppm? Does that also affect residential pools in the US? Looking at our target FC, that would basically limit CYA to 40ppm for non-SWG pools and to 70ppm for SWG pools. During a SLAM, FC would be above the 5ppm limit for CYA>10ppm. That 5ppm limit certainly is a bit of backpedalling in acknowledging HOCl as the active sanitizer. I'd really like to understand what the reasoning behind that is.
This is exactly the disconnect and why I am very slow to offer advice to pool owners not seeking it. They say, " a pool service is cheaper and I don't have to think about it". It is like having a yard guy or a house cleaner they say. They refuse to see the subtle clues of a water imbalance and when they can no longer ignore problems they refuse to associate them with their flawed methodology and instead double down on more chemicals with fancy names. 
