The -30% value is taken from the peak value, but doesn't look at the total amount posted per day.
So I took the data from the blackout.photon-reddit site source.
It seems that it makes a Reddit Api call every Minute searching the newest Post and Comment and calculates both per Minute rates.
I wanted to see the effect the Blackout had over the day, so I summed the data and plotted it: Seems like between 11th and 12th June the comments/day diminished by -19.2%. The posts/day saw a decline of -8.9%
The sub with the most Activity was probably Askreddit