Balance is ultimately relative, insofar as it is purely defined by the ratio of front length vs. back length (ie. neckpoint to chest line in both cases). If you add to one, you change the ratio in the same way as if you removed from the other. So, it doesn't matter if you add & subtract, or just add, or just subtract, so long as the net balance alteration is the same.
Obviously, this is ignoring the change to armscye depth, pocket heights, etc. which occur when you move one balance instead of the other. But these are simply adjusted to compensate. For example, if you shorten back balance by 1cm, it is generally expected that you would also lower back armscye by 1cm too.
To further illustrate this point - balance can also be altered by 'passing up' the side seam so that the relationship between the chest & waist balance marks is used to assess the total change in balance. For example, to lengthen back balance, you sew the side seam together so that the back waist mark is higher than the side panel waist mark. This has an identical effect to changing the balance at the shoulder seams. It is in fact the preferred method of altering balance in circumstances where you do not have enough inlay at the shoulder seam to implement the required alteration.
Balance is ultimately relative, insofar as it is purely defined by the ratio of front length vs. back length (ie. neckpoint to chest line in both cases). If you add to one, you change the ratio in the same way as if you removed from the other. So, it doesn't matter if you add & subtract, or just add, or just subtract, so long as the net balance alteration is the same.
Obviously, this is ignoring the change to armscye depth, pocket heights, etc. which occur when you move one balance instead of the other. But these are simply adjusted to compensate. For example, if you shorten back balance by 1cm, it is generally expected that you would also lower back armscye by 1cm too.
To further illustrate this point - balance can also be altered by 'passing up' the side seam so that the relationship between the chest & waist balance marks is used to assess the total change in balance. For example, to lengthen back balance, you sew the side seam together so that the back waist mark is higher than the side panel waist mark. This has an identical effect to changing the balance at the shoulder seams. It is in fact the preferred method of altering balance in circumstances where you do not have enough inlay at the shoulder seam to implement the required alteration.