No exact measurements required. Just get a good handful and put in down through their back feathers. Work it down through under the wings and around the underside. Then pick the bird up and tilt forward. Most birds will through their tails up exposing the vent. Dust the area good and work it down through the fluff under the vent and up through the tail.
This is easier with 2 people, but 1 can manage. If you are sensitive to dust, wear a mask and gloves. I don't and am congested for a couple days afterwards, but that is just because I am stubborn(and not very bright).
When finished sit the bird down in their house or an area that they dust bathe in. They will immediately shake off excess. No sense in wasting it.
Matt