In theory a Semi-Ellipsoidal head is simply half of an ellipse rotated about its axis. A 2:1 SE head is based on the long dimension of the ellipse being twice the short dimension. Since the head is based on half of an ellipse, the height winds up being one quarter the diameter. You'd have to use the basic equation for an ellipse and take the derivative to get the curvature at any point.
Or, as doct9960 pointed out, VIII-1 UG-32(d) lets you aproximate.
Depends on what you're doing. If you're using a CAD program to draw the vessel, you should be able to draw the ellipse exactly and cut the "inside" half off. This approach is probably faster than calculating and drawing a large diameter circle and two small diameter circles, placing them all on the tangent line, aligning the small diameter circles to match the diameters, and trimming the excess parts.
jt
