Well the OP wrote:
So, if at sea level, with 65% power set at 23"/2400RPM, and 20GPH -- what do I need to set the manifold to if I wanted to keep 65% power (and 20GPH) with 2400RPM at 8,000'?
The constraints don't actually make sense, as 20 GPH at 8000 ft with an equivalent mixture setting will give more power than 20 GPH at sea level. If you allow the mixture setting to be varied, to maintain 20 GPH by enriching the mixture, then there's a continuum of MP vs mixture settings that will meet those constraints.