First sketch a curve by the command, Sketch, Curve
Then Sketch a point On Curve Using the Length Radio Option
Then pattern this point taking advantage this value (length radio)
Sketch the rib taking reference (in Skecher) only the point that you make first
Then pattern the rib using e reference pattern option
Then look at the result.
Remember that when you sketch a point on curve with a value, value =1, makes the Pro/E to put the point at the end of the curve and if you put a value lower than 1, you are closing to the start of the curve.