Problems encountered in the practical application of the three-table method for alignment

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Dear teachers, I have been looking for information on the three-table method for alignment online recently. As I am learning this technique, there are a few points I don’t understand, and I hope you can help me clarify them. The factory is currently undergoing major repairs, and the alignment work of the units is also in progress; this is a good opportunity to thoroughly learn about alignment. 1. In the actual alignment process, the axial values are S1=（s1’+s2’’）/2, S2=(s2’+s2’’), S3=(s3’+s3’’), and S4=(s4’+s4’’). Some people on forums say that it should be S1=（s1’-s2’’）/2, S2=(s2’-s2’’), S3=(s3’-s3’’), and S4=(s4’-s4’’). How are the plus and minus signs inside these parentheses determined? 2. In the final calculation, Y1 = b*L_front/D = (S3 – S1)*L_front/D; some sources state that Y1 = b*L_front/D = (S1 – S3)*L_front/D. How should we understand the subtraction when the values inside the parentheses, (S3 – S1) and (S1 – S3), are swapped? Is it possible to have scenarios like (S3 + S1) or (S1 + S3)?