OK, tape the end of a string to the floor, put your 2X4 about 3' from where you taped the string end. Hold a pencil at this point and use the string to make an arc. If the arc is too small move out the string and try again. When you get a nice arc that will fit on the side of the 2X4 mark it with your pencil.
I made all my bending forms this way.
Marin, you are reading way too much into this process, bow making isn't an exact procedure, you have a lot of wiggle room however you do things. All of bow students I tried to teach the craft to who never finished a bow were all afraid of doing anything on their own for fear of goofing up, I suspect you fall into this category.
Eric, over and out.