It's best to look at voltages instead of percentages when preparing a battery for storage (Except for the BoardBank). A good storage voltage is about 3.4V - 3.7V for a lithium ion battery cell, or a 1S pack. Let's take a look at how this translates to a full pack.
Example: Onewheel XR
A Onewheel XR battery pack is built with a 15S configuration. That means there are 15 cell groups working in series to deliver the amount of voltage the device needs to be properly powered. Winter is coming, and we'd like to store our Onewheel until we can ride again.
To get a range of what the XR pack should be at, we can simply multiply the voltage range for a 1S by 15, to get the range for a 15S pack.
3.4V * 15 = 51V
3.7V * 15 = 55.5V
Therefore, the storage voltage for a 15S pack, like the one in the Onewheel XR, would be anywhere from 51V - 55.5V.