You want to raise the chlorine high enough so that when you test in the evening, it is not below 5. Try going to 7.5 and if you still drop lower than 5, try 8, etc. Actually, with a CYA at 90, you might add enough bleach to take the chlorine to 10ppm and then you might not need to dose it for a couple of days. That is one advantage of having a higher CYA pool ---- less frequent additions of bleach. You just have to learn how your pool behaves. By the way, it is best to add bleach in the evenings when the sun is off the pool rather than in the mornings.