calculate front/back ratio of Yagi antenna?
 

I was looking at some designs for a 3-element Yagi.
It seemed to me that the ratio of the reflector length to the
director length is approximately the ratio of the highest
frequency to the lowest frequency of the band you wish to
receive. But I gather the front to back ratio is more difficult
to compute. Is there a formula that can tell you what the
FBR is, given the lengths and spacings of the elements?