I registered PoolSolutions.com in 1996 . . . where were you?

Anyhow:

1. Read the Best Guess chart page, linked in my blue signature block. It answers the questions you are asking.

2. Get a K2006 testkit; it's nearly impossible to manage a high CYA pool without one.

3. Changing 10% of water at a time is VERY inefficient. If you can do so safely, changing 1/3 or more at a time is much more effective. Of course, if you have a vinyl or FG pool, it's not safe to drain.

4. When you get the K2006, test by mixing 1 cup of pool water with 1 cup of tap water, and then testing the mix. Multiply your CYA test result x2

5. You CAN operate a high CYA pool, simply by using high chlorine levels. There are actually many advantages to doing it this way.

6. Since your pH, TA, & CH are OK, it's OK to ignore pH readings for awhile. 5 ppm FC will NOT kill algae when you have 100+ ppm CYA. Buy a cheap OTO test kit (yellow drops). Add PLAIN 8.25% bleach, 3 gallons at a time, till you reach the DARK yellow / ORANGE-ish yellow OTO level (FC > 15 ppm). Don't go higher till you have the K2006. But, at this level, you should begin to at least control the algae.