NavList:

I measure index error not index correction. The reason I do it this way is I sometimes use the marine method for handling this error and ai somethimes use the flight navigation method. The two methods differ in whether you apply corrections to Hs or to Hc. because the sign is opposite in the two methods. If I were to lable it "index correction" I might forget if that was in relationship to marine for flight navigation methocology. 

I also posted a method for doing observation correction in which you allows ADD the error which is less likely to introduce errors caused by having to subtract several corrections in a row. Take a look at this method and the form to use:  http://fer3.com/arc/m2.aspx/new-way-apply-sextant-corrections-LaPook-jan-2011-g15459