N1 is one of the power indications used with a turbine. However it isn't necessarily the best indicator of how much power is being used. Since it measures the compressor speed, it will change with density altitude. The higher you go in temp, and/or altitude the higher your N1 will be for a set power setting.
The one that is used most often is the torque gauge. It measures oil pressure at a set of gears usually in the main rotor transmission, and is a real indication of how much power is being transmitted through the transmission. The guage is either marked in %, or in p.s.i. You will have a maximum (red line) and that is all the power that you have available (turbine temp, and N1 allowing of course). So if the a/c requires 75% to hover, and you have a maximum of 100% torque allowable, you have 25% in which to use for departing. Depending on the a/c, at least 15% torque is required to vertical out of a confined area.
Hope this helps clear it up.
Cheers
Randy_G
