Which percentage of One for All can Midoriya manage without hurting himself by the time he awakes his black whip quirk? So Midoriya can't use 100% of One for All because he hurts himself. But he's been improving the percentage of it he can use without hurting himself, first it was 5% IIRC, then he increased to 10% and he tries to keep improving.

Which percentage of One for All can Midoriya manage without hurting himself by the time he awakes his black whip quirk?
Midoriya was able to use 100 percent of his power even before awakening his black whips and without hurting himself. 

As I stated in my answer here, 


   this was made possible by Eri's power so Mizuku's body remains unhurt while unleashing 100 percent of his power.


Also, even before this, with the aid of Melissa, he was also able to use 100 percent of his power via the Full Gauntlet, albeit he is limited to three smashes. 

Without any aid, the current limit for his One for All is 15%, as stated here.