The manual does state you should start with fully charged battery.
When they are part discharged it seems to be difficult to accurately assess remaining charge especially when you suddenly apply load e.g. to take off.
It's probably OK to take off with part charge more or less the same day yu used half the battery but don't try anything too ambitious.
If the charging won't start because the battery is above 70% try turning it on then attach the charger. 

In storage these batteries like to sit at about 50% or irreversible chemical changes occur which reduce performance markedly.
So they have an inbuilt system preset to gradually discharge to that level after ten days if you don't use them.
In the app you can reduce reduce this to (say) 3 or 4 days to extend life - or try to run to 50% before storing anyway.
Note this also means you don't want to put them away much under 50% - so if you ran it way down, ideally put it on charge for a few minutes when you get in. 

I find in practice you've often more or less done what you want before 50% is used anyway, if you have 2 or 3 batteries it's easy enough to pop the next one in without running it below the level where you could just leave it in the case after use