practical best range application?

I understand the theory vis a vis lowest L/D TAS and the power
available/power required chart, but let's say you're in flight in a normally
aspirated prop plane, your destination is blown up, your alternate is
unservicable for some reason or other and you need to stretch the fuel you
have the farthest distance possible.

What are the quick and dirty steps to working out the optimum alt and speed?
I mean in the cockpit, not in the comfort of home.

Thanks in advance.