If you are adamant about using a wax pencil, put the tip on the point you want to draw a line from, then slide the edge of your compass up to it on the rough bearing then rotate and if necessary slide the compass round the point until it is lined up with the grid lines, then draw the line. I used to do it that way when drawing on perspex map displays. I did it backwards too to it could be read from the other side. We had to write backwards and do map references backwards too.