In short the instruction for drawing the curve are as follows.
Please note that the height is considered to be the intended height and bleeds should be ommitted in these calculations. When extending the curve to into bleed area, it should be extended along the circle in bleed areas above the rectangle, or in a straight line along the tangent of the circle in bleed areas below the rectangle.
Following is a step by step guide for drawing the curve
Draw the bar with corners A, B, C and D (clockwise, starting at the top left) in which you want to incorporate the curve. Define w = |AB| = |CD| and h = |BC| = |AD|.
Mark the point on the bottom line of the bar where you want the red area to end on the bottom. Call this point E.
Mark the point F on (the extension of) the bottom line of the bar, at a distance of h×4 to the right of E.
Draw an arc x (left side of area β) on the circle with center F and radius h×4, starting from E ending at the top line of the bar.
Mark the point G on the bottom line of the bar, at a distance of h×0.15 to the right of E.
Mark the point H on the bottom line of the bar, at a distance of h×0.15 to the right of F.
Draw an arc y (right side of area β) on the circle with center H and radius h×4, starting from G ending at the top line of the bar.
Fill the area enclosed by AB, x, CD and AD (area α).
Fill the area enclosed by AB, BC, CD and y (area γ).