This paper may be helpful: Methods for in vitro evaluating antimicrobial activity: A review - ScienceDirect
...and this one contains details on the calculations: (PDF) Protocol to Evaluate Antibacterial Activity MIC, FIC and Time Kill Method | Fadwa Alshareef - Academia.edu
'To obtain the time-kill curve the bacterial strains growth rate must be counted at different time intervals starting from 0 hours to 24 hours, and these time intervals are plotted as a semi-log plot or graph. The bacterial strains would be usually plotted on the or-dinate (Y axis) in logarithmic scale while the corresponding time is on the abscissa (X axis) in arithmetic scale.'
I believe that you need to already know the MIC when performing a time kill assay.