Either will work, but the FC levels from that page are more "conservative" than the pool calculator. Those levels are from this page which is ChemGeek's expanded CYA/Chlorine chart.
The levels in the chlorine chart are higher than pool calculator's, by a small amount at low levels (up to 40ppm CYA), a wider range at mid range levels, and then they are actually lower than the pool calculator at very high CYA levels.
They both work, and the general rule of thumb around here is that either is fine but if the lower values from the pool calculator aren't working, try the higher levels from the chart. I wish I had a better explanation of it all, sorry. It's a little confusing to me, still. However I have been assured that both sources are valid. I choose to stick with pool calculator for simplicity sake.
I would do the 20ppm personally. Maybe find a happy medium and shoot for 24ppm?
The levels in the chlorine chart are higher than pool calculator's, by a small amount at low levels (up to 40ppm CYA), a wider range at mid range levels, and then they are actually lower than the pool calculator at very high CYA levels.
They both work, and the general rule of thumb around here is that either is fine but if the lower values from the pool calculator aren't working, try the higher levels from the chart. I wish I had a better explanation of it all, sorry. It's a little confusing to me, still. However I have been assured that both sources are valid. I choose to stick with pool calculator for simplicity sake.
I would do the 20ppm personally. Maybe find a happy medium and shoot for 24ppm?