Some discussion on power instruments:
The only true power instrument for a turbine is the torque, which measures the twisting forced the engine is applying to the transmission. As the rotor bites into the air, the twist must increase, so the torque rises. Torque times rpm is horsepower.
Several torque sensprs exist, the old fashioned kind uses the oil pressure inside the engine to push the helical output gear back into place (helical gears try to walk along their axle in exact proportion to the torque they are transmitting, so the push-back on them is a reliable way to calculate torque.
Most modern engines have a torque tube, where the inside is connected from engine to transmission, and subject to full torque, which slightly twists the shaft. An outside cover is attached at one end, the other end floats above the shaft, and does not transmit torque, so it doesn't twist at all. This outside cover has some bumps on it, as does the regular shaft. When at zero torque, these bumps all align evenly. When the inner shaft winds up with a bunch of torque, its set of bumps shifts over and gets closer to those of the outside cover, so the once even spacing is now a series of unevenly spaced pairs of bumps. We place a magnetic pickup just above the bumps, which sends out an electric pulse every time a bump passes by. As torque rises the pulse pattern shows the shift of the bumps, which is proportional to the torque. The instrument system converts this pulse shift into a reading on the gage.
For most helos, takeoff power is easy to estimate, using the ratio of the power needed to hover. If you can hove at two feet with x torque, then you can make a running hover takeoff (a really flat one) with just 1% torque above that. If you need to make a towering takeoff, you need OGE power plus about 2% more. OGE power is about 1.15 times IGE power, so about 1.17 times IGE power will let you make a vertical takeoff. For example, if you indicate 60% torque to hover IGE, then it will take 71% torque to make the towering takeoff (60X1.17=70.2)
Alladinsane (cool username!) has a fine rule for those times when you can't land and hover first. The only warning is that the slightest descent rate or deceleration while checking the 80 knot power will make the estimate very liberal, and the true performance disappointing, so do the check carefully. I have made similar rules of thumb for Black Hawk and S-76, but usually fell back to a quick reference chart I distilled from the flight manual, based on the operating area and season. A simple table with a small band of altitude (say zero to 5000 feet in 1000 foot intervals) and a small band of temperatures (say 30 degrees C in 5 degree intervals), altitude along the side, temp along the top. Inside each square, put the exact weight your machine can carry at that condition for IGE and OGE, based on the charts in the flight manual. Determine the actual empty weight, plus you and your crew, plus equipment, and subtract all that from the flight manual chart IGE and OGE hover weight. As you fly up to the landing site, note the alt and temp, look up the weight you can carry on your handy chart, subtract your fuel, and bingo, you will be within a few percent of what you can pick up.
A comment on engine temp and N1 as power gages. They do produce information proportional to power, and are very useful, but they are not necessarily very porportional, especially N1. This means that a few tenths of a percent N1 can change performance all out of proportion to the indication. Torque is very linear, and easier to interpret. Both temperature and N1 are limit values that must be respected, and both can be the power limit on a gioven circumstance, of course.