That's a REALLY good question:

If your pool is still green then there is no way your FC is 47. Sorry MBar, but you don't figure FC from what you add, but rather from what you measure.

When you are fighting a serious algae bloom you'll use lots and lots of chlorine.

Your pH reading cannot be taken accurately unless your chlorine level is down. If you are using Taylor R-0004 or R-0014 to test your pH, it will be more accurate because it neutralizes chlorine up to about (I think) 15ppm.

If you have chlorine neutralizer drops (Theosulfate, Taylor R-0007) you can add 2 drops to the pool water in your test cell before adding the pH chem.

You COULD bleach your liner if your FC is sustained higher than 15 for a while, but if you reach 15 to 20 in an algae-infested pool it probably won't stay there long enough to damage or even fade your liner.

Still, you need to be able to measure your chlorine at the higher levels, especially now. You need a FAS-DPD chlorine test kit. You can get it in the PoolSolutions.com kit Ben sells--the PS-234, or the Taylor 2006, or Leslie's FAS-DPD Chlorine Service Test Kit, but only on-line.

All three will sell you, I believe, just the FAS-DPD kit.