Bob Bob wrote:
"Work out the path loss based on the basic distance formula and plug in
your power losses, and RX sensitivity to determine your margin and max
range."

Add in the antenna gains. You`ll need them to offset path and
transmission losses.

Margins involve choices. Performance and reliability depend on those
choices. I often choose to imagine the earth with only 2/3 its actual
diameter for my path profile. Then I use 0.6 of the first Fresnel zone
for added clearance at the path grazing point. You need 30 or 40 dB
excess signal into the receiver under normal conditions to make the
receiver very quiet and to allow margin for path fades if you need high
performance and freedom from fades.

Path loss is computed from frequency squared times distance squared with
constants as needed for the system you are working with. Someone has
usually worked up a chart of path loss versus distance for the frequency
you are working with. Every time distance doubles, loss increases about
6 dB.

Best regards, Richard Harrison. KB5WZI