If you truly have a CYA reading of 0 (and with opening to a brown quagmire and having not added any additional CYA this season, it's very possible to have a 0 CYA reading), then yes, you'd use the first line of Ben's Best Guess Chart. The key to the whole "getting rid of the green" process is consistently keeping the chlorine at "shock" level until it clears up and you have a CC reading of 0 and your chlorine level holds overnight. At that point you can then let the chlorine level drift down to the "minimum/maximum" range. Consistently means testing at a minimum of in the morning and at sunset and adding chlorine to bring the levels back up to shock range. If you can do it at noon or mid afternoon also, that would be even better.