Carl,

To keep my message short, I didn't give all of the details. With the following conditions:

pH: 7.5
FC: 3.0
CYA: 30
The following don't have a much smaller effect on HOCl:
Temp: 80ºF
TDS: 550
The following has no effect on HOCl:
CH: 300

You get 0.045 ppm HOCl and 0.048 ppm OCl for a total of 0.093 or about 0.1 ppm total for HOCl and OCl-.

I have been in communication with Ben on the detailed spreadsheet I created and plan to start a thread in the China Shop (perhaps this weekend) to start the discussion on getting real-world experiences from people. Though I am confident of the chemistry determining HOCl levels at various levels of CYA and FC and am also fairly confident of the 0.011 ppm HOCl level that is the minimum for disinfection (of easy bugs, not hard-to-kill ones), I am not at all sure what the proper levels of chlorine are needed to prevent algae nor to properly shock algae (and will likely vary by type of algae). This is where some real-world experience could help determine these values.

The spreadsheet is complicated and solves the chlorinated cyanurate equations iteratively since an exact solution requires solving a quartic equation (or ignoring a very minor species, a cubic equation) and even then you need to iterate to solve for the ionic strength influences on activity. Nevertheless, there is an approximate formula you can use so long as your CYA ppm is at least 5 times your FC (the formula really falls apart terribly below a ratio of CYA/FC of 3).

(ppm HOCl) = (ppm FC) / ( 2.7*(ppm CYA) - 6.6*(ppm FC) + 5.2 )

and if you are interested in the FC for a given HOCl (to construct the equivalent of Ben's table, for example), you can use the following which just solves for ppm FC from the above.

(ppm FC) = ( 2.7*(ppm CYA) + 5.2 ) / ( 6.6 + 1/(ppm HOCl) )

The constants in the above formulas are for a pH of 7.5 (which is the only parameter that significantly affects these constants). With the spreadsheet I can easily calculate the constants for other pH, but remember that the above formulas are approximate. For example, with FC of 3 and CYA of 15 the formula gives HOCl as 0.117 when the correct answer is 0.107. That's not bad (about a 10% error). However, with FC of 5 and CYA of 15 the formula gives HOCl as 0.400 while the correct answer is 0.246 (about a 60% error) which isn't very good.

Richard