Calculating target heart rate for cardiac improvement

Many people wish to improve their cardiac ability but when they step onto a piece of equipment such as a treadmill, bike, eliptical, etc. they just start exercising, and largely ignore their heart rate and the sophisticated devices installed on the equipment to monitor their HR during the workout. You have probably heard it before, but here it is again: Cardio exercise conducted at 70% - 85% of your maximal heart rate is the target range for improving cardiac ability.

So how do you know what this range is? Here is the equation:
For the lower limit (what you should not let your heart rate drop below) can be found by: .7 x (220-age)

For the upper limit (what you should not continuously let your heart rate go above) can be found by: .85 x (220-age)

So, with these equations we can say for example: A 54 year old man wants to improve his cardiac ability. He should stay between (.7 x (220-54)) = 116.2 BPM and (.85 x (220-54)) = 141.1 BPM.