megan,
That is a good system, I used to teach a similar one, what you are doing is estimating the shape of the power required curve from the Vx to the OGE power. The ratio is probably about 2x, with the OGE power as close to twice the "bucket" speed level flight power.
Another rule is to perform the max power pull at the proper conditions of altitude and temperature near the landing site and note the maximum torque that is available. Subtract 10% and use that just below max power as your never exceed. Make a slow approach bleeding airspeed deliberately and increasing power deliberately and watch the torque. Do not keep your speed up and rely on a hairy cyclic flare at the bottom, or you might be surprised by the power suck-in and bottom falling out. Instead, keep slowing and progressively watching the power rise. If you can get close and slow and still maintain that margin you observed, then good on you. If not and the power is clearly going to be close to the limit, just lower your nose and accelerate away from the landing, all under full control.