I think that where FED mentions 200ft/min ROC OGE probably refers to the `standards of the day` from AvP 970,BCARs, and MilSpecs;a lot of what was written in them was rather vague,but changing, and test methods/instrumentation as well. WIWO WW10s,we didn`t have a performance manual,as such,that came much later,(I think !) and one had to rely on fuel-flow as an indicator of power.Digging out my old notes, even though one had an idea of what one could lift,we would do a power-check,at 50kts,level flight ,near the LZ,check power available,either f/f,ptit,or Ng and then decide either the type of approach and landing,ie OGE,OGE+ 2.5% thrust margin,10-15ft-roping,low hover,or a run-on landing,if taking troops/freight in,or the amount of t/frt one could lift out.
Later, the`rules-of -thumb` were combined on the kneepad checklist when torquemeters became standard,to give one both t/o and landing performance data.
Does anyone use a similar method these days,or is FADEC/HUMS your friend ?