Well, first you estimate your age (+/- 2 hours). Then subtract your
resting HR from your age and multiply by the square of your red *** cell
count (this will give a good indicator of your O2 carrying capacity). Then
add to it your tidal volume of total expired air (after an interval
workout, of course). Subtract the date, time of conception, and finally
your femur length which should give you the best indicator ,in the most
scientific fashion, of your exercise potential. Or, you may have
calculated the sex of your nextborn. Maybe you could just guess fom when
you start to breath hard and would just like to stop and eat a ring ding.
That's how I do it.