To blend the neck into the body of the bottle you might want to use a cubic equation like y = ax3+bx2+cx+d. Although you could use a sine function or some other similar bending function.
I had Derive find the coefficients a,b,c,d so that the graph passed through (1,1) and (3,2) and such that the derivative y ' = 3ax2+2b+c equaled 0 at x = 1 and x = 3. It was amazingly easy. Check out the attached Derive file.
