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George Huxtable, many years ago, said that a simple arithmetical average of the sights was just as accurate as curve fitting and a lot less trouble. I have always done this. The justification is that the sights are taken at roughly constant time intervals, so the weighting on each value is about the same. 

For the round of sights in David Burch's paper, the workings for the averages are given below:

​And the result can be plotted as the red square on David's graph on page 2

​As you can see, the average sighting lies a little above the straight line through the three 'good' sights, influenced as it is by the 'outlier' second sight. 

Now, only one sight reduction needs to be done, which saves a lot of time when you are using 'short' sight reduction methods.

Geoffrey Kolbe