SAS et all, best sustained as requested by the OP is referred to in the OEI rate of climb chart in the Section 4 Performance that you provided.
70 KIAS at 100% rotor RPM. I believe that is fairly close to Vy. Depending on aircraft weight and Hd, you may need to use MCP, even though the best rate of climb at that speed (or any speed for what matters), is achieved with 30 minutes OEI power.
Thanks for that .pdf BTW, it did bring back quite some memories.