Take 220-age(43)- your RHR (resting heart rate) = X

RHR should be determined by measuring it as soon as you get up in the morning.  Count the number of beats in 60 seconds (don't cheat by doing only 30 sec).

Next take X and:

X * 60% + RHR = Low end of zone
and
X * 70% + RHR = High end of zone

For example:
A 31 y/o male with a RHR of 43
220-31-43= 146

146 * 60% + RHR = 131

146 * 70% + RHR = 145  so my zone is 131-145

If your RHR is 58.

220-43-58=119

119*60%+58=129

119*70%+58=141

Here are some other notes to think about:

The Energy Efficient or Recovery Zone - 60% to 70%
Training within this zone develops basic endurance and aerobic capacity. All easy recovery running should be completed at a maximum of 70 %. Another advantage to running in this zone is that while you are happily fat burning you may lose weight and you will be allowing your muscles to re-energies with glycogen, which has been expended during those faster paced work-outs.

The Aerobic Zone - 70% to 80%
Training in this zone will develop your cardiovascular system. The body's ability to transport oxygen to, and carbon dioxide away from, the working muscles can be developed and improved. As you become fitter and stronger from training in this zone it will be possible to run some of your long weekend runs at up to 75%, so getting the benefits of some fat burning and improved aerobic capacity.

The Anaerobic Zone - 80% to 90%
Training in this zone will develop your lactic acid system. In this zone your individual anaerobic threshold is found - sometimes referred to the point of deflection (POD). During these heart rates the amount of fat being utilized as the main source of energy is greatly reduced and glycogen stored in the muscle is predominantly used. One of the by-products of burning this glycogen is the runners worst enemy, lactic acid. There is a point at which the body can no longer remove the lactic acid from the working muscles quickly enough. This happens at an individual heart rate for us all and is accompanied by a rapid rise in heart rate and a slowing of your running pace. This is your anaerobic threshold or POD. Through the correct training it is possible to delay the POD by being able to increase your ability to deal with the lactic acid for a longer period of time or by pushing the POD higher.

The Red Line Zone 90% to 100%
Training in this zone will only be possible for short periods of time. It effectively trains your fast twitch muscle fibers and helps to develop speed. This zone is reserved for interval running and only the very fit are able to train effectively within this zone.

Resting Heart Rate
To determine your resting heart rate (RHR) is very easy. Find somewhere nice and quite, lie down and relax. Position a watch or clock where you can see the second hand. After 20 minutes remain where you are, do not sit up, and determine your pulse rate (beats/min). This is your RHR.

If you have a heart rate monitor then put it on before you lie down. After the 20 minutes check the recordings and identify the lowest value achieved. This will be your RHR.

As you get fitter your heart becomes more efficient at pumping blood around the body. As a result you will find your resting heart rate gets lower so you will need to check your RHR on a regular basis (e.g. Monthly).