The added piece should be long enough to include the roughing-in and finessing of the dips(fades)... which is beyond the flares.
So if, for instance, you have a 4" handle, 1 1/2" flare (on each side), and extend the dips 1 1/2" (on each side) beyond the flare... that's a total of 10".
Depending on design, I have made that piece up to 14-15" long... but never shorter than 9".