A-D,it would take a few hours in the classroom to show how the power figures are derived and put into the Performance graphs,but perhaps Shawn and Matthew could elaborate at some point on new test methods.Several decades ago I flew Whirlwinds(turbine S-55) and our
only `power` indication was fuel-flow,Ng,and PTIT,before a suitable Tqmtr was developed.The FFmtr also had a `correction scale`,for both altitude and ISA deviation.We would do a power check in level ,stabilised flight at 50 kts(min. power speed/just below best ROC speed) and note FF,then pull max `power` to determine the limit,either FF,Ng,or PTIT.Depending on the amount available,would determine the type of landing one could do,ie OGE,IGE,wheels touching,or run-on,using a `formula`;it could also give one (using another `formula`/rule-of thumb) how much one could lift out in terms of running T/O,cushion creep,200ft/min ROC ,etc. Hwever, it should be remembered that the fuel-flow in-hand (or Tq,or Coll.pitch )at min pwr,is not the same as Tq/ff/cp. in the hover. It also gave a measure of `power` of the particular airframe,in terms of a good ,or crap set of blades,or worn compressor,in this case in tropical conditions.
So,if you can get time etc you can make up your own `formulae`,by preferably in a ballasted a/c,do some hovers at various hover heights,noting `power`,and then at min power speed/best ROC/power available.
Obviously,in certification there is a lot more work/time spent to derive the graphs,to which one should always use !!,however,reality is always different,and manufacturers (and their lawyers) don`t offer the `quick` ways..Hope that helps, but if you want any further info,send a PM...
So,you can do your own set of `formulae`