Being pedantic this evening, let me weigh in....
The exact quantity of the standard is subject to questions in its own right, since it is nowhere near 10% for careful procedures where flat approaches and departures can be made. In many types, basic HIGE power can get you off the ground successfully, if a slight dip at 12 knots on takeoff is acceptable.
That all being said, it is 10% of the hover power, so that your 50% power to hover would rule of thumb to 55% as the standard. Each percent equates to a bit of margin above that needed to hover, typically 5 to 10 pounds of extra lift for each extra horsepower. Knowing the calibration of the torque gage is nice, so that you can derive the exact contribution of each percent as equivilent weight or thrust margin.