So if I have to replace 75% of the water in the pool, how is doing this method any less expensive then doing a series of draining and filling? I've already drained probably 10-12" out of the pool and replaced the water. I'll continue with this new method tomorrow though.
You can calculate the amount of water exchanged - I have to imagine your pool is at least 3’ deep, but let’s say it’s only 3’ deep. That’s 36”. 5” would be 14% of the pool. That would bring your CYA from 150 to 130. So you do it again - now it’s 112. A third time would take it down to about 96.
If you can afford the water to do it this way, and the patience, great, but it would take 9 exchanges to get it down to under 40 CYA - and that’s if you have a 36” deep pool. Most likely it’s deeper and that number goes way up.
See above. Draining in small steps means the pool water is mixing as you go and that means each drain is subsequently less effective.