G B Z, I'm not really sure what you're saying but I take it that it has something to do with a flat map of curved surface??? If this is the case I'm not sure that a distance of 38 miles would show up an error of 3 to 4 Deg.
I'm phyically drawing a line from the centre of the VOR and extending it to the waypoint. Then with the protractor I measure the true heading and add the variation to it.