I think the manual stated it clearly... Drain it until it's pretty much dead. This is the same for RC cars or any rechargable battery. They have a "memory" if you want to call it that. If you only drained it 80% and then recharged it, you'd never get full use out of that battery ever.
This really counts for the first time you use it before you charge it.
I'm not an expert though.
Which Tablet do you own?