when you don't plan to use it for a while, discharge it to around 40%

It is true that the optimal STORAGE charge level for Li-Ion is about 40% of full charge, but in practice this is hard to do.
I suspect most Pearl users would use the device often enough for this to not be an issue.

If you WERE to store the Pearl for a good while, I would suggest fully charging it, then run it down about half way, and store it in a cool, dry location. Then fully charge it before starting to use it when you put it back into service.

I would expect most users to fully charge the device when they have the opprotunity, use the batteries as desired, then charge it again. This would be the model the designers would expect most people to use, and with a properly designed Li-Ion battery management system, you should get good lifetime and capacity from the battery pack.

One note; if you run the batteries to exhaustion, it is a good idea to recharge them ASAP, as fully discharged batteries (Li-Ion and other types) will internally degrade if left in a discharged state for an extended period of time.