Hi Eric,
Remember the formula:
((1,000,000/pv) * (conc) * (L) = ppm

1,000,000 = one million
pv = Pool Volume in Liters
conc = bleach concentration AS A DECIMAL, so 6% is .06, 5.25% is .0525, and 12.5% is .125
L = amount of bleach added, in Liters.

So if you have a 10,000 liter pool, and add a liter of 6%, the increase in PPM is....6!
If you have a 20,000 liter pool and add 1 liter of 6%, the increase in PPM is 3
15,000 liters, 4.5.

Using this trick I just reduced PV to 10 and L to 1ml 1 ml is 1/10000 of 10 liters, but 6% of THAT is 6/100ths of a milliliter, or....6 millionths of the 10 liters, ie, 6ppm.

BTW, I do all my pool calculations in gallons of water and bleach, but it doesn't matter. The ratios are the same.

So, if the % is 10.8, but the measured is 10.3ppm, do you REALLY need to care? I defy ANYONE to measure their pool's current volume that accurately.
Think about it: If your pool is 40'x20' and rectangular, a difference of just ONE INCH in depth is 500 gallons (almost 2000 liters). One inch. How many let their pool drop a few inches before adding water?

So...my method is a nice quick and easy way to determine the pool store that sold you 12.5% LC really sold you old junk that's barely 5%. If it's close to 12.5, you are good. What's your alternative? Hire a lab to test it for you? Seems pointless.

Finally, the ONLY risk of overdosing your pool with chlorine is you MAY bleach a vinyl liner. All the calculations in the world don't change the recommendation that when adding chlorine, adding a little more than you calculate is a GOOD thing, but with everything else you want to add LESS than you think you need and re-measure.

Carl