With the connectors at both ends already determined the only open question was the length. I’ve pretty much decided to mount the voltmeter under the dash shelf so, armed with a ball of twine, I went out and worked out a routing. So the answer to: “How long should the fly lead be?” is: “As long as a piece of string.”
Making up the fly lead was the same as making up the voltmeter connector but with the gender of the connectors inverted. A wrap of the wires with black insulting tape to keep things tidy and it was finished.
The final QA check was to mate the voltmeter connector and the fly lead and, using a multimeter, check the wiring end-to-end.