NavList:

" I teach my students to ignore index error, since what we
use is the index correction. They are opposite, and that can be
confusing."

Jim,

I agree with you on this for the paperwork reductions with given information, but when checking the sextant for alignment, you had better know how to read the micrometer drum to know if the error is on or off, and having read it; knowing whether the correction is to be added or subtracted.  I think I may be missing something, but you can't just drop the whole concept of on and off the arc errors in practical navigation.

Jeremy