It is important to remember that it occurs after an uptrend and usually marks a major trend reversal when complete. While it is preferable that the left and right shoulders be symmetrical, it is not an absolute requirement. They can be different widths as well as different heights. Identification of neckline support and volume confirmation on the break can be the most critical factors. The support break indicates a new willingness to sell at lower prices.
Lower prices combined with an increase in volume indicate an increase in supply.
The combination can be lethal and sometimes there is no second chance return to the support break. Measuring the expected length of the decline after the breakout can be helpful, but don't count on it for your ultimate target. As the pattern unfolds over time, other aspects of the technical picture are likely to take precedence.