The flu can have a very dramatic effect on your heart rate. Do not consider any readings taken while being sick as your resting heart rate. To get most the accurate reading, wait until you are feeling better, than lie flat on your back for at least five minutes. It is important to get an accurate reading of your resting heart rate, as it's just as important for calculating your training zones.
To determine to maximum heart rate, the most accurate way is to be tested in lab, but that can be expensive. The poor man's way is run a very even paced 5k, then sprint like hell for the last 200-400 meters - whatever will get you across the line completely worn out.
I know one person suggested running an all out mile. That may work for you, but most people need to run over 10 minutes to get their hearts up to maximum. The problem is that your legs will fill with lactic acid before your heart has a change to reach its maximum.
To calculate your zones correctly for running use the Kavorian (sp) formula. It goes like this ...
Target = Min + ((Max - Min) * Effort)
This is your heart rate reserve. Here's an example of a person who has a resting heart rate of 50 and a maximum heart rate of 200, trying to run at 70% effort ...
Target = 50 + ((200 - 50) * .70) = 155
Warm-ups and cool-downs should be down between 60 and 70 percent. Base mileage should be done between 70 and 80 percent. Lactic threshold sessions should be done between 80 and 85 percent. Anaerobic repeats should be over 85 percent by the end of the repeat.
When doing base mileage, warm-up and get your heart rate up to 70%. Then, stay at that pace for the duration of the run. As your run continues, your heart rate should increase slowly due to heat, dehydration, and fatigue. Thus, you may need a fudge factor for hot, humid days depending on how well your body deals with heat.
Hopefully, this is helpful without too many spelling errors.