The valve length that you want for the rear disc wheel will depend on how much cutout space the wheel has. That said, latex tubes typically come with a 42mm valve, so that is probably what you would want to try first.

For an 80mm wheel, you would start with the 42mm valve on a latex tube and add an extender to get about 15mm outside of the rim so that your pump can get a good grip on it. Simple.