what percentage of battery safety limit Dji phantom 3 is still fit for use? 80%, 70% or under?

I can't answer that, but can tell you that you can probably "recondition" it.  After sitting a couple of weeks, mine slipped a little to 95% or less, so I did the deep cycle "recalibration".  I charged it up to 100% and took it out to fly, never getting far away, but exercising the battery until it brought it down to my 'warning' setting.  At that point, I brought it in front of me (home point), and let it hover until it got to the critical point.  I ran it a little bit longer and then landed.  I did a full stop landing so it wouldn't try to lift off again.  Then I restarted it and let it idle until it shut itself down.  At that point, I let it cool a minimal amount of time.  It was warm, not hot, but I didn't want it to sit for any length of time in a discharged condition.  As soon as it was fairly cool, i charged it up again.  Both batteries were done, and both are still at 100% life and seem to work well.  They just don't like to sit for lengths of time.  Most battery designs these days seem give the best life when used regularly.