Camera.WorldToViewportPoint math

Not sure if this (graphics) is the proper forum, but it seems closest to my aims.

I'm trying to jobify a piece of my code that checks whether an object is in the viewport of a camera.
To do this, I need to get the matrix used in WorldToViewportPoint(Vector3 position) (as I can't use the Camera-class itself in a job).
The code is 'hidden', and I can't seem to get any of the matrices that I can pull from a camera (e.g. ProjectionMatrix, WorldToCameraMatrix) to get me the same result as the method.
Can somebody tell me how I would implement WorldToViewportPoint using matrices I can pull from the Camera?

 

roughly

Code (csharp):

1. Vector4 worldPos = new Vector4(position.x, position.y, position.z, 1.0);
2. Vector4 viewPos = camera.worldToCameraMatrix * worldPos;
3. Vector4 projPos = camera.projectionMatrix * viewPos;
4. Vector3 ndcPos = new Vector3(projPos.x / projPos.w, projPos.y / projPos.w, projPos.z / projPos.w);
5. Vector3 viewportPos = new Vector3(ndcPos.x * 0.5 + 0.5, ndcPos.y * 0.5 + 0.5, -viewPos.z);