Normalization of data in PRISM

I have data from three time course experiments. I want to get the mean value of each time, and then to normalize this mean of each time (5min, 15min, 30min and 60min) to the mean of the 0min (having standarddeviations for all times). When I click on "Analysis" and --> "normalize" they ask how 0% and 100% is defined. What could be defined by 0% and 100% in my experiment?