Shawn...I guess until the industry demands such knowledge, it will not be given. It would be my guess that it has to do with liability---but I'm sure you knew that huh?
As for rule of thumb..I use the mountain flying technique that I learnt from Larry Doll, (former chief instructor at Rocky Mountain Helicopters). This technique supposedly works for all Bell products---I routinely land my L4 upto 10k and occasionally upto 12K during fire season and it works.
On final approach, flare the aircraft to a slight nose up attitude, (be slow and methodical with most of you power already applied), to be a little short and 100ft above any obstacles on your path---hold the slight flare till speed falls through ETL and you will feel a slight shudder, level, apply the last of the power and note:
1. Pedal position
2. N1
3. TOT
To hover IGE it will take an ADDITIONAL:
1. One inch of left pedal
2. 12% N1
3. 35 degrees TOT
To hover OGE will take and ADDITIONAL:
1. One and Half inches of left pedal
2. 15% N1
3. 50 degrees TOT
IF you do not have the additional pedal, N1 or TOT available before hitting any red line limits, you are in a position to nose forward and fly away.
If you have the available pedal, N1 and TOT, you can continue the approach in a nice controlled manner all the way to the ground. The mark of a perfect approach is to hold the collective in one position, the aircraft will settle all the way and "kiss" the ground and then lift off again to about 2 feet as ground effect kicks in.
I have tested this "rule of thumb" at most weights, lots of Alt/Temp variations and it works for me. The biggest thing to learn, if this is not your standard procedure, is the slow/methodical approach.
As for the original question of having enough power to get out---this comes down to being at "one" with you aircraft and understanding all the performance. I am lucky in that I fly the same aircraft most of the year---I know all of her quirks. By using the above technique, and having a cheat sheet in the aircraft, I am able to make a reasonable determination of whether I can get out or not.