Precise Curves?

Hi,

I am fairly new to flash,

I am looking for a method of using the pen tool to draw geometrical curves. Say I wanted to create a button with a square shape which I then add curves to on its left and right side, I mean I don't know what the value of the curve I draw on the left side would be compared to the right so how can I keep my shape geometrical? what I'm looking for is an option for guides that could maintain a corresponding angle for both sides in order to be precise when drawing curves with the pen tool.

If anyone could possibly help me I would greatly appreciate it.
Thank you

Capo