You don't need to drain; you just need to adjust your chlorine levels to compensate. Use this chart as a guide:

Ben's Best Guess Chlorine Chart

But, you will need a DPD-FAS chlorine test kit (drops based DPD test) in order to stay on top of things. As far as I know, you can either order a Taylor K-2006 (what my kit was based on) or the TFP approximate copy of my PS234 (defunct) kit.

Ben / PoolDoc