What countersink depth measures
Omni's countersink depth page describes the depth as the distance from the material surface to the bottom of the conical countersunk section. It is what lets a flat-head screw sit flush or slightly below the surface.
Countersink depth formula
depth = (diameter / 2) / tan(angle / 2)
The fetched page includes a compressed formula block, but its own worked example makes the intended relationship clear: divide the countersink radius by the tangent of half the included angle.
Source example
Omni's FAQ uses diameter 0.55 m and angle 75 degrees. The resulting countersink depth is about 0.358 m.