Lets do some math.
DeferredLightUniforms.SpotAngles.x is cos(OuterCone).
So equation is:
cos(OuterCone) / sqrt(1.0 - cos(OuterCone)^2) which simplifies to cot(x) when x is positive. Cot(x) is 1 / Tan(X) which is standard perspective correction scale term. I don’t see any reason why 0.5 is there.