It doesn't. Some evaluation has to be subjective, as not everything can be measured. How do you measure comfortable or fluid? Where people set the line of not comfortable/fluid to comfortable/fluid varies dramatically. Hence introducing measurable criteria of trim (X degrees or less from trim) and no changes in depth beyond a specified range, and possibly within a specified time limit.Whatever.
In the end it really doesn’t matter.
Can anyone think of any other way to set objectivity to performance requirements?