I sing with a small schola, only on special occasions: we are not attached to a parish.

When we chant psalms for the Office (usually Vespers) we usually do this:
* a "cantor" sings the antiphon up to the "*"
* the whole schola sings the rest of the antiphon
* the same cantor sings the first half of the first psalm verse
* the cantor together with "his" half of the schola sings the rest of the verse
* the "other" half of the schola sings the next verse
* and alternately
* until the Gloria Patri and the Antiphon which we sing full

Sometimes we do something else.

And we don't consider ourselves constrained particularly on this point. But I feel sure I've read somewhere either guidelines or even real rubrics about how to assign a cantor, cantors, halves, and full to the liturgical chants. Who can help me find or re-find those guidelines or rules? And how constrained are we?

Thanks for any help.