When I was replacing water last year to get my cya down, as soon as it reached the skimmer so that it wouldn't suck air, I started my pump to recirculate and filter the water until it got up to mid skimmer then started the next drain. Making sure I turned off my skimmer valves so it just drained from the bottom drain. I also used backwash to drain-- no reason to not clean the filter in the process. If you have a vacuum, you might want to make sure that the boost pump doesn't start while you're draining. Unfortunately, I hadn't thought of that and I locked up my boost pump irreparably. Remember though I was going to do 5 drains so I didn't worry about testing my cya until after the the drain. I figured draining 3 ft in the shallow end of your pool would be draining about 11,500 gals, so you really do need to retest before you start your second and probably final drain. That should tell you about how many inches you need to drain for it to get you to 50. Though, I'd figure it will be close to 3 feet again. But that's why you also need to know a more exact capacity for you pool.