Lets attack this from a theory perspective. Flying at an airspeed that results in the minimum power required for level flight, Vy, for the DA and aircraft configuration including Nr will provide either:
a. The largest power margin, if power required is less than power available AEO or OEI. or
b. The minimum rate of descent, if power required exceeds power available AEO or OEI.
Should power available and power required be equal then level flight is possible. If power required is less than power available then the aircraft can climb, fly faster, fly slower or a combination of those. If power required is greater than power available then a descent is inevitable.
The RFM will provdie the accepted manufacturer data to determine these numbers. The cruise chart for the appropriate DA would a good place to start. This chart will show the Vy, and resulting power required, for the DA and aircraft GWT and the maximum power available. Should the maximum power available not be on the chart then this figure will also be published in the RFM.