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Index correction is easy to measure as long as you have an unobstructed horizon at sea. You simply look at the horizon and adjust the sextant so that there is a single line across the horizon, and then read the vernier setting on the sextant.
I used the Navy Mark V bubble sextant for navigation in the deserts of Arabia when I did expeditionary travel in the deserts. I never applied a bubble correction to my sites, and I found that I was genereally two to three miles off my gps position using star sights at twilight.
After a 110 km desert run, I generally found that my DR position was off by about 10 kilometers since we were not travelling in a straight line. I used my DR position as an assumed position to compute my sights at twilight, and I was usually 2-3 miles off the GPS position using the bubble sextant without a bubble correction being applied.
From my experience, I would ignore the bubble correction and not worry about it. The important thing to me was to be able to work the site and get within a couple of miles of my true position. Unless you are surveying with a transit on a tripod, it's going to be difficult to know your position within more than a couple of miles using celestial navigation. And when you get on a yacht at sea, you will be very lucky to be within two miles of your gps position in my experience. Of course, if you are on the deck of a freighter taking sight, you may be able to get closer than two miles.
I would pay attention to the index error, but I would not worry about the bubble correction.
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