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0.5 x 10° x sin(60) = 4.33°, conversion angle.
You should use 5 degrees here instead of 10. It's obvious if you draw a picture of the situation with 2 parallel lines representing the mentioned lines of longitudes, like on a mercator chart, with straight rhumb line between the points and a downwards pointing great circle line. The point where the g.c. is most south is in the middle between 20W and 30W, so 5 degrees longitude.
Even then I am not sure how accurate this answer is in the real world..
Back in the days we didn''t have this q. in the database. (pipe-smoking smiley)