MonkeyBusiness

The cash drain ratio is the ratio of currency held by individuals to funds deposited in the banking system.

You are interpreting is as the ratio of currency held by individuals to total excess reserves, which is incorrect.

With a 50% cash drain ratio, if there are $100 excess reserves, 33 will be drained from the banking system, and 66 will be deposited in the system. 33 / 66 = 0.5

If the cash drain ratio were 100%, then $50 would be drained from the banking system and $50 would be deposited in the system. 50/50 = 1.

Hope this helps