Accurately calculate a lens diameter with ease using this Excel Lens Diameter Calculator. Complete with a diagram to illustrate each part of the process and full instructions including explainations of each measurement. 
 
All you need is the frame size and mono PD. The horitontal decentration is calculated together with a total resultant decentration if vertical heights are provided. 
 
Two lens diameter results are given.
 
The first utilises the effective lens diameter, HBOX and DBL if you know these measurements from a traced shape. This provide the most accurate results.
 
The second utilises the typical frame measurements (Frame Width and Bridge Width).