As the head tube is at an angle I don't think it important to worry about the much smaller angle between chainring tooth and dropout, especially as it will change when you rotate the fork. For an easier, more accurate method without removing the fork try this:

Use a T-square or level (or really any long piece of metal, plastic, or even a piece of stiff cardboard, at least 2 inches wide with a 90 degree corner) and lay the long edge along the center of the fork blade (and head tube if long enough) so that the end intersects the dropout center. Measure from the long edge to the center of the dropout. Done.