The HRmax method measures your heart rate to estimate how hard you’re exercising.
There is another method called Rate of Perceived Exertion (RPE) which is based quite simply on how hard you think you’re exercising, which, in a number of studies, turns out to correlate fairly well with your actual heart rate.
The RPE range is from 6 (at rest) to 20 (maximal exercise). That number, multiplied by 10, roughly corresponds to the heart rate for that level of exercise. If you rate your exercise at a level of 12, for example, your heart rate would be approximately 120 beats per minute.
This means if you wish to exercise to an intensity level in the middle of the range for your age you would aim for a level of 14 if you’re 30, and 12 or 13 if you’re 50.