Thanks for the feedback!
John, it sounds like I should clarify the 'sometimes slightly'. When we have room to back up we'll do that in order to keep the LZ in sight, but in some takeoff directions you have buildings behind you as well and not very much room for backing. Unfortunately, our flight manual doesn't go into elevated helipad ops, and I haven't found any performance charts relevant to the situation. If it makes any difference, I fly a 212.
Geoffers, I'm not familiar with the "WAT curve", at least not by that name. What exactly is it that you're referring to?