Unity world curve shader graph, how to start curving from a specific distance?

Before you feed the z position into the Power node, subtract your minimum curving distance from the z to get a signed difference, and then run that through a Max node to take the greater of 0, or that z difference. Then pipe that clamped z value into the Power node.

If your camera is looking in the -z direction, then you’d do something similar, but add the minimum curving distance and then run it through a Min node instead. Either way, what you’re doing is moving the vertex of the parabola forward or back along the z axis, then clamping everything closer to the camera to zero so you only see the further branch of the parabola.