heya tdw after some test with the sextant i have see that is a -8 degree error in angle ( sextant angle measurement is less with 8 degree )... i have made this test and compared with stellarium ....
EDIT
my bad sorry i didn't calculated the time btw UTC and and my location time
take a look here i made the first tutorial using your sextant
http://www.subsim.com/radioroom/showthread.php?t=206103