Topband: Rule of thumb for Gamma match?

[Carsten Esch]

Hi,

is there a rule of thumb on spacing and dimension (in terms of diameter) 
of a Gamma match to match a tower for 160m?

The tower we want to try to use for 160m is about 196ft high, basewidth 
is 18.5ft, at 196ft the width is around 5ft.

Thanks,

Carsten, DL6LAU