(Click on the image to enlarge)
(1) Dr. Rao's book p. 49 (Wiley-Liss, New York, 1993)
(3) Further simplified formula: (Narang R, et al. Cardiology 1997;88:271-276)
Effective Balloon Diameter = 0.82 (D1 + D2)
Some images worth learning from are posted here (Note: Most of them are plagiarised. But, sources are referenced)