When checking the errors of the sextant, the third adjustment for Index Error, using the sun is found by:
• setting the arm of the sextant at approximately 32on the arc and bringing the true sun above the reflected sun. Adjust the two images until they are ‘limb upon limb’, then note the reading (say 28on the arc);
• re-set the arm at approximately 32off the arc and bring the images again Limb upon Limb, and read the sextant again (say 37off the arc).

Take the difference of the two readings and divide by ‘2’ to give the Index Error, and call it the higher of the two.

Example -
37'-28' = 9'
9/2 = 4.5' off the arc (Index Error)