If the earth is spherical and colat is 30 deg and angle subtended at the pole is 20 deg then the two points at lat 60 are 597.71nm apart. I've got that. Using a calculator provided by
http://www.naco.faa.gov/index.asp?xm...online/compsys and WGS84 spheroid the distance is 600.29 I've got that too. But isn't the apex of the great circle found to be at latitude 60d 22.74m? Does that not indicate the max cross track error twixt rhumb line and GC? I seem to remember the red Admiralty book Navigation had a formula for finding the apex in general (and difficult) cases. Bear in mind that I am brilliant at getting hold of the wrong end of the stick.