This is all you need to do (t get things started). If the results weren't satisfactory, the something else was wrong, ie: drift, blade tension, fence misalignment, operator error, etc.
As far as the method of drawing two lines from the corners and finding the intersection, this should be as good as your Starret Gauge, except of course, this method is only accurate for parallelograms and perfect rectangles, not all quadrilaterals. Don't you have the same problem with the Starett anyway (pencil line width, etc)?