After many a hard hour of pain staking practice, ive finally got it - i think.
If you want a smooth shape instead of the "potatoe" you mentioned Flex, you have to focus on your left thumb pivot position.
It is also essential that you keep it on the edge of the deck, and nearly half way between both bottom corners, contrary to common belief that it will be bigger should you have it on the corner. In fact this ensures that the fan can actually be larger as it gives you a wider ark to work with.
I also found a very helpful tip on the grip with your fingers. Take your left index finger and position the tip of it on the back of the deck, on the furthest bottom corner from your palm. Then the top left corner of the deck should rest in the notch between your fingers and thumb. Therefore, when you do the fan you can do half with a singular motion of your left hand moving down, and complete the other half by doing the same with your right.
I watched and read a hell of a lot of tutorials to find out the best way and i think what i have written above is an easy and good way of doing it.
Steve