40v Battery - how far should you drain them when using?

Just got my first 40v: the 750 CFM blower. Charged up the battery to full and tried it out.

Kept checking the battery level while using it. Got to the point where the last of the 4 green lights would blink when I pushed the charge light button.

Is that a "uh-oh, you drained it too far" or a "okay, this is when you should stop draining me" indicator? Not sure if I should stop when I have one solid green light or the blinking one.

Thanks