Buck wrote:

...
Does anyone know where I might find that formula?


Buck:

Some designs for the "ugly balun" appear to vary, sometimes widely.

I don't know how a fellow could go wrong if they take into the reactance
of the coil (to the freq/freqs in question) and make that at least 5X
the impedance of the coax in question. At the higher freqs, I prefer a
10X ratio, 160m makes the 10X ratio cumbersome.

Others may provide info to suggest other routes ... or more desirable
solutions ...

Regards,
JS