Yep,Alex's method works. In case Delta Wun Wun's explanation isn't completely clear - your two vertical lines need to converge towards the pole. And it may help to put in your rhumb line track as a curve on the equator side of the great circle (hint from Baz, Alex's right hand man: "rum comes from hot countries, ie near the equator"!). Then you have the whole picture and it should be obvious whether to add or subtract; hope so anyway, but if this just confuses the issue then ignore it.