[Elecraft] Mathematically challenged

[gehringc at charter.net]

I want to try and build an HF two Element Vertical Antenna for 20 meters. 

While there are several java scripted web-sites that will calculate and provide the physical measurements for the Driven, Reflector elements and spacing.  

I would like to use my MJF-259 to complete the fine tuning during construction and given that the frequency of the Driven element will not be the resonant frequency of the reflector.  

What would a good equation be to determine the resonant frequency for the reflector based on the desired frequency for the Driven element?

Some references that I have found state the Reflector is approximately 4% longer than the Driven and the Director is 4% shorter, with element spacing 0.15 to 0.25 of the desired freq.  I would like something a little more specific.  Not that I can build it to spec or that the MFJ259 will be that accurate, I would still like to work with figures that provide something other than approximate.

73 K2CG
Chuck G.