Ok imagine a circle with a little circle inside it. Now imagine a triangle between the 2 circles. now picture one side of that triangle and draw a line through the centre of the inner circle parallel to your triangle side.
Now there should be a point where the tip is highest relative to your triangle side of rim of the outer circle.
now draw a parallel line across that point parallel to your triangle side. From these lines your triangle side should have a ratio where it is between the inner and outer line. so you plot a line between the two circles at this ratio and do the same for the other 2 sides of the triangle and the line you have Plotted is a curved off triangle.
In 3D you could put a tetrahedron between 2 spheres. this time though you'll have to reference a triangle across the middle of the inner sphere and across the tip of the highest relative tip on the sphere surface.
This can be scaled to model a head very well.