Next move (ctrl-left mouse button) the light blue dot either above or below the horizon at the front or back corner of an imagined box/room. You can also rotate the crosshairs (alt-left mouse button) to have a slanted horizon line like the camera's tilted on its side.
Where it is horizontally doesn't seem to matter much, but the vertical position is vital to place correctly now since moving it later will move the entire ruler instead of repositioning the horizon within it. The first thing you'll want to do is move (ctrl-left mouse button) the black crosshairs on your horizon line. I could not locate any tutorials on the subject so I figured I'd make one as soon as I figured it out. Paint Tool Sai 2 has perspective rulers and perspective grids, and I found the 3 point perspective ruler super confusing and unintuitive.
