Oops, poorly worded.
Here's the situation that brought the question to mind- Watching a utility guy check his aircraft's performance, post maintenance. The test was a long-line lift of X weight.
However, this test was accomplished with a good breeze, 10 knots gusting to 18, at the surface. At the top of the becky, the aircraft would have been above obstructions to wind. The pilot's no fool and knows his next lift might not be in a comparable breeze, and was wondering if there was any way to predictably compensate for the ops check being performed in ETL, beyond the principle that the rotor's more efficient in ETL and thus the bird's stronger.
I know of no rule of thumb, could find nothing comprehensible to a mere pilot. More efficient, powerful, whatever, in ETL is insufficient data, smacks of "ther be giants here".