@Joozey, yeh that was a mistake, but like you said it doesn't seem to change anything.

@Quadraxas, thanks that seems to improve it. I can reach any part of the grid now. The only problem is that it only works if I don't move the camera, but only rotate it. From the start position, the tile seems to keep in sync with the mouse position, but if I change the camera position then it starts to become offset from it.

I guess this is dependent on the distance from the plane, so any ideas how I can use the result from the c_trace, to make the dragging proportional?

Thanks again guys.