Might be addressable by solving two scenarios. The first is a highly-risk-averse approach to meeting your minimum needs, the second would be a risk-indifferent approach to investing anything left over. To put it into the coin analogy if your minimum need was $11 but the guaranteed rate is $12 you could take the $12 but then immediately flip again with the left over dollar and get an average of $1.25 with it, for a total return of $12.25.