The forearm is composed of many muscles. Some people work some and not others. My index finger, middle finger and thumb were stronger in my right hand (manipulating art stuff) and my ring and middle finger were stronger in my left (carrying). It's probably that your natural way of using your hands doesn't call your middle finger in to work too much. Get some grip-trainers and do lighter farmers' walks to build the muscle up. Also consume animal tendons and ligaments (in sausage-form, if you're ridiculously squeamish) to give your body materials to strengthen your own tendons.
Perfection is entirely individual. Any philosophy or pursuit that encourages individuality has merit in that it frees people. Any that encourages shackles only has merit in that it shows you how wrong and desperate the human mind can get in its pursuit of truth.
I get blunter and more narcissistic by the day.
I'd apologize, but...
If you are trying to give your heart a workout without doing cardio then dont worry about %s. heavy walks, pushes and pulls all work the entire body including the heart and lungs. I am very anti cardio in that i dont see it having much benefit and many negatives. I think you can get very impressive results with just strength endurance work alone.
Things like high rep work, complexes, intervals, walks, pushes, pulls, will accomplish this nicely.
I try to add one of the above to almost every workout as a warmup or finisher, occasionally it will be an entire workout.