If you've got a double crankset, you measure to the middle area between the two chainrings. If you've got a triple crank, chainline is measured to the middle of the three chainrings. You should measure from the centerline of the bike (so, out from the seat tube). Don't worry about measuring the sprockets - the measurements that I listed earlier are set up for 130mm-spaced road 8/9/10 speed cassette hubs, or 135mm-spaced off-road 8/9 speed cassette hubs.
You've got a road double, so the distance between the center of the seat tube and the center between the chainrings should be 43.5mm.