This has been covered before but lets go with it. Because the bike I ride is so small it is uncomfortable and less powerful for me to ride with the balls of my feet over the pedal spindle. When I ride I just let my feet find the best position and go. Doing this I find that my right foot is on the heel and my left foot is rinding mid-foot. Now we have been told for decades to ride with the ball of the foot but why? It seems if you position the cleat mid-foot you have a more solid base for pedaling.
What kind of pedals are you using that you are able to pedal with your left foot on your heel?
I ride a horrible cheap ass wal-mart bike so the pedals ar platform.