Hi Kes, thanks for the reply, it's nice to meet another UK forum user!!
My numbers all have some degree of uncertainty. If I calculate the volume of the pool based on given measurements, I actually get 80,900 litres, rather than the 90,000 litres that I was previously told. I put 80,900 in the pool calculator. There was already a few ppm of FC in the water, and so that plus the 10 litres of sodium hypochlorite should have taken me to 22 ppm or very close. Of course now I am wondering if the pool dimensions that I was given are exact - I'll have to get the tape measure out at some point, I suppose.
The test was done with 8ml deionised water and 2 ml pool water - it was my carelessness that I wrote 5:1 when I meant 4:1 and multiply by 5. The pipette that I used isn't terribly accurate, so the error in the final result could be substantial, like more than several ppm. I did another test at 50:50 dilution in the evening (24 hours after first addition) and got 10.78 ppm FC.
I haven't been out this morning to test - the pool is 1 mile away, and I haven't decided on what to do next. Combined chlorine is still up, so I guess I'm not finished. However, I don't have any algae to fight, over the last week the pool was finally holding it's chlorine levels overnight, and I know there's a good chlorine residual in the water, so I'm hoping that time delay isn't critical - although I suppose I'll be losing more chlorine to sunlight at higher levels, and will have to make up for that at some point.