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? I don't understand how's that possible? I mean zones 6-8 if zone 5 is your race pace. All you need is to know your maximal heart rate (MHR). If you don't have any special devices you may count it by the formula MHR=220-"your age". Ex. if your age is 30 then 220-30=190 is your MHR. Of course this value is approximate and if you need correct value you need special equipment. Ok, now knowing your MHR you may count heart rate (HR) for each zone. I zone - 60-70% of MHR - aerobic and recovery II zone - 71-75% of MHR - aerobic and power III zone - 76-80% of MHR - glycolysis IV zone - 81-90% of MHR - anaerobic threshold, overload endurance V zone - 91-100% of MHR - anaerobic training, lactate tolerance and lactate production (both are maximal pace, the difference is in amount of rest you get)

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? I don't understand how's that possible? I mean zones 6-8 if zone 5 is your race pace. All you need is to know your maximal heart rate (MHR). If you don't have any special devices you may count it by the formula MHR=220-"your age". Ex. if your age is 30 then 220-30=190 is your MHR. Of course this value is approximate and if you need correct value you need special equipment. Ok, now knowing your MHR you may count heart rate (HR) for each zone. I zone - 60-70% of MHR - aerobic and recovery II zone - 71-75% of MHR - aerobic and power III zone - 76-80% of MHR - glycolysis IV zone - 81-90% of MHR - anaerobic threshold, overload endurance V zone - 91-100% of MHR - anaerobic training, lactate tolerance and lactate production (both are maximal pace, the difference is in amount of rest you get)