Buck, N4PGW wrote:
"Does anyone know where I might find that formula?"

Look at the ARRL Antenna Book. The 19th edition (old) has a choke balun
for low-frequency antennas on page 6-9. The text says:
"In general, an isolation choke inductance of 50 to 100 microhenries
will be needed for 3.5 and 1.8 MHz, ground-plane antennas. One of the
easiest ways to make the required isolation choke is to wind a length of
coaxial cable into coil as shown in Hig 18. For 1.8 Mhz, 30 turns of
RG-213 wound on a 14-inch length of of 8-inch diameter PVC pipe, will
make a very good isolation choke that can handle full legal power
continuously."

On page 11 of Terman`s 1955 opus is found formula (2-2):
Inductance in microhenries = F n(sqared) d

F is a constant that depends on length/diameter. There`s a chart in the
book for F.
n is the number of turns.
d is the diameter of the coil measured between the centers of the wire
in the coil.

The purpose of the coil is to present a high impedance to currents on
the outsifde of the coax so quick and dirty is OK so long as there is
enough impedance. You may get away without a coil form at all. You might
lose some of the isolation that coil length provides so I don`t
recommend just coiling coax like garden hose.

Best regards, Richard Harrison, KB5WZI