Calculating a number for the intensity of a workout, how?

I am writing my workouts in excel. How do I calculate a number for the intensity of a swimming workout? Is there a standard way of doing it? I'm using zone 1 to 5, where 1 is recovery and 5 is racepace. Additionally I have three more zones that are lactate tolerance (zone 6), lactate production (zone 7) and power (zone 8). How do I calculate all theese zones and distance into one number, that gives an adequate impression on the intensity of the workout?