for an ellipse, you take half the major axis on the compass, and from the edge of the minor axis you swing an arc. Where it crosses the major axis (two points) are the focal points. In a parabola, since the line of eccentricity is always 45 degrees, it's the same distance as the vertex is from the eccentricity line, so you can just use that distance from the vertex.

macmilkman— 21/06/16