Nick,
Nice idea on the chart construction.....years ago in the hills of Iran and other high hot places where we had to operate from sea level to 14,000 feet msl.....I oft times would use my V toss or Vbroc airspeed and establish a 500 fpm rate of climb in still air....and note the power required. Seems hover power (IGE) usually produced about that same performance in most aircraft I have flown thus if I had the power to climb at 500 fpm at the same altitude as the landing site in still air.....things seem to work fine. The big trick was to maintain the correct airspeed, rate of climb.....and be in still air.
The Alouette III and Lama used a pitch meter to calculate such things and made it very easy......it used OAT, Altitude, aircraft weight as variables to determine the pitch required/available for the data entered......very much nicer setup than any US made aircraft I have flown.