While I'm not a newcomer to magic I figured that this particular flourish qualified for this part of the forum.

Here is my question. When I do a thumb fan(with the faces/backs of the card facing me) all is well, the display of the fan is symetrical. However when I turn my palm face down to display the backs the backs are not in the shape of a fan. I hope this is clear.

Could anyone offer any tips as to how I can do a thumb fan and show both sides as a symetrical fan?

Thanks for your help.

Mostly a little more practice, and learning to control the pressure. It can be frustrating at first, but give it time your fans will get more uniform and better looking.