My CYA is at 100. The pool calculator says to replace 60% of my pool water to get at a 40 reading. I have a 14,000g fiberglass pool that I have never drained before, except for your basic backwash. Do I drain all at once or a little at a time? Will the pressure from draining all at once crack the fiberglass. Or should I drain it 40% to get a reading of 60?