Excellent Guidance!

My tap = 1.2ppm (did the high precision 0.2/drop measure)
My 10l & 1ml sample (blended and 10mins) = 11.5ppm (0.5/drop)

Bleach concentration = 11.5-1.2 = 10.3ppm = 10.3%

I guess that's not too bad. I never realised the conc could drop so much. I'll have to check again in a month, just to see the rate. There could/will be a point where other options would be more cost effective...depending on my consumption.

thanks Carl!