If you are singing then the correct position is standing. Once you stop singing you may sit or kneel as appropriate.
This is the general principle behind the rubrics. When singing, with the exception of specific reverences, one always stands. This is why at a Sung or Solemn Mass, the faithful should not be kneeling during the Sanctus or Agnus Dei, but wait until after the singing is finished to kneel, since by right, they are supposed to be singing these. When a polyphonic Mass is used, they should kneel immediately, since they are not singing. This also explains why during the Asperges or Vidi Aquam, only the priest and servers kneel for the incipit (when the priest sprinkles the altar) and the faithful remain standing throughout without genuflection.
This is one general principle, and a brief study of the rubrics, or a read of various rubrical authorities, will demonstrate the various principles that lay under the specific rubrics. It is more profitable to understand these general principles than to have the rubrics memorized, since principles can be broadly applied.
Regarding the choir:
The faithful may not have specific rubrics, but there is not a huge room for variation.
When we say that the faithful follow the choir, we are speaking of clerics sitting in choir, not the schola, nor any non-liturgical choir. It is true that there are not specific rubrics for the congregation, but there are fairly clear rubrics for clerics in choir. There are a few areas where local variation and custom have a legitimate part, but one should also remember, in the United States there are already customary norms (which existed from before the introduction of a new form of Mass) and those should be followed as opposed to making up one's own "local custom" in each parish.
What is typical at a Sung or Solemn Mass:
The men who sing the Propers stand during these (except when specifically required to kneel -- e.g. 2nd Alleluia during Pentecost Octave)
All stand for the Kyrie, Sanctus, Agnus Dei and Ite Missa Est
The faithful may sit during the Gloria and Credo after the celebrant does.
The faithful follow the words sung at a Sung or Solemn Mass, not the recited words -- thus they genuflect when the choir sings "Et Incarnatus est..." not when the priest recites it. And they ought to, more properly, follow the Introit that is being sung, not the prayers at the foot of the altar.
The faithful sit or kneel during the Propers as appropriate.