"What I don't understand is why your published performance data aren't good enough, especially in a training environment"
Leaving aside whether you have a min spec engine, and if your weight is calculated correctly, none of this tells you what the effect of down flowing or up flowing air is at your landing area. Assuming a training environment, where weight is not be added to leave the confined area, you want to shoot an approach to the imaginary platform above your confined area, and before you slow below translational, determine what torque/temp/gas producer margins you have to get in and back out. Depending upon the up flow or down flow, you may be able to take more or less than the charts say.
Mel Schiller, instructor extraordinaire at Canadian Helicopters, has a whole program on confined areas developed there, that I can't do justice to, that combines wind finding, checking power and assessing the spot. This is just part of the whole procedure. It starts with the high recce, where he looks at 4 S's: size, shape, sun, starting point. Then he goes to the low recce and wind finding, the overshoot and option to land, maneuvering in the confined area and the departure.