This column is the discharge at the cv boundaries conditional to the injection time \(t_{in}=0,\ q\left(t,0\right)\).  They can access also the additional information, coming from She table that  the total input at time \(t=t_{in}=0\) is equal to \(P\left(0\right)=s\left(t=0,t_{in}=0\right)=20\)  (and must be equal to the total sum of the terms appearing in column for  \(t_{in}=0\),  performed after the concentration time).
Thus, They can then be properly normalise the column by subdividing all by \(P\left(0\right)\). In fact the response time distribution, is defined by: