Since your cya is 90, you're going to be putting alot of chlorine in the pool in the next week. I'd just do a partial drain/refill to lower cya. It should be more cost effective unless you have water restrictions locally and could get fined! If you could get your cya down in the 30-40 range, you'd only need 3-5ppm residual, not 15. Also, you won't have to babysit the pool for a week... much easier in my opinion.