Consider a _Capital Centered Marxism_, which would carry no "policy option," but build its own "policy" GIVEN Marx's Critique of Political Economy, which as Postone points out is not the same as a "Critical Political Economy." It offers us an account of an "ideal average" of all possible specific capitalist systems -- seen as history, not subjected to any "moral" judgment. (See Gáspár Miklós Tamás: Telling the truth about class http://www.grundrisse.net/grundrisse22/tellingTheTruthAboutClass.htm )
" Marx does not 'oppose' capitalism ideologically; but Rousseau does. For Marx, it is history; for Rousseau, it is evil."
Marxists then do not inherit any particular policy. For myself, I derive my "policy" primarily from Rosa Luxemburg, whose "socialism or barbarism" is not a slogan demanding socialism; it is an historical analysis indicating that the future is invisible, that barbarism is as likely as democracy. (And of course that is all we have had since 1914: Barbarism.)
And incidentally, it is not necessary to be a Marxist to recognize Luxemburg's alternatives.
St. Thomas would have had some answer to your questionas to an optimal interest rate. I doubt that Marx can help you.
Carrol