The easiest way to remember the storage voltage is to look at the battery - it has the voltage you need printed right on the battery!
The storage or nominal voltage is the most stable voltage for a LiPo battery to sit unused. Always recharge your battery to the storge voltage (the voltage on the battery!) after use. And then charge to the maximum capacity just before use.
Search this support section for more information on your specific battery. Type in "voltages" or "Charging instructions" and select the articles specific to your battery - "original" or "plus".