#but haven't seen anyone point out the thirdway points trick

[alizardjae]

Finding halfway and thirdway points in perspective

Start with your rectangular plane in perspective, know your vanishing point. For the purposes of this tutorial we will be assuming only one vanishing point per plane

Draw an X connecting the corners of your plane; the intersection point is the center of your plane

Draw a line that runs through both the vanishing point and the centerpoint to find the halfway points of your vertical lines. Draw a line that runs through the centerpoint and perpendicular to the horizon line to find the halfway points of your horizontal lines.

Both the X and the halfway points on the perimeter can be used to divide your plane into thirds

To find the third of the way points for your horizontal lines, draw a V shape from the corners of your plane to the halfway point of the horizontal perimeter lines

Draw lines perpendicular to the horizon line through the intersection points of the V and the X from the halfway point finding part to find the third of the way points for your horizontal perimeter lines

To find the third of the way points for your vertical perimeter lines, draw a V shape from the corners of your plane to the halfway point of the vertical perimeter lines, then draw lines running through the intersection points of the X and the V as well as the vanishing point

Hope this was reasonably worded, please ask for any clarification.