It's not a matter of accurate, but of speed. A higher chlorine level will kill the algae more quickly and oxidize chemicals more quickly. The TFP chart is set so that there is the same active chlorine level at all CYA levels. Ben's chart was done more from experience than from calculations so varies from this. You can certainly use lower numbers for shocking, but it will take somewhat longer. Once the algae is dead, most of the time of clearing a pool is with circulation/filtration to remove the dead algae.