You could probably write out an equation that would take the distribution of decisions (expressed in payoff for defection vs. cooperation), and compute the optimal probability distribution (as a function of expected value for defection, or ratio of cooperation/defection payoff, perhaps) to use for this strategy.

Bongo's comment rightly implies that the result will depend on whether other players observe the probability you will betray them as a function of payoff, or just the probability overall.