Heart rate reserve is used to calculate

It's maximum heart rate minus resting heart rate, and it calculates optimal (target) exercise heart rate, so you don't work too hard or too little.

The average resting heart rate (hrrest) for a 34 year old is 73 beats per minute2. 34 year old males average heart rates of 71 bpm while females average slightly higher at 76 bpm2. the normal resting heart rate range for all adults and children 10 and over is between 60 and 100 beats per minute3. 90% of 34 year olds resting pulses fall within the range of 54 to 92 bpm. in general, an adult's resting heart rate will be lower for those in better athletic condition.