  Determining compositions of low-mass exoplanets is essential in understanding
their origins. The certainty by which masses and radius are measured affects
our ability to discern planets that are rocky or volatile rich. In this study,
we aim to determine sound observational strategies to avoid diminishing
returns. We quantify how uncertainties in mass, radius and model assumptions
propagate into errors in inferred compositions of rocky and water planets. For
a target error in a planet's iron-mass fraction or water content, we calculate
the corresponding required accuracies in radius and mass. For instance, a rocky
planet with a known radius error of 2% (corresponding to TESS detection best
errors) demands mass precision to be at 5-11% to attain a 8 wt% precision in
iron-mass fraction, regardless of mass. Similarly, a water world of equal
radius precision requires 9-20% mass precision to confine the water content
within a 10 wt% margin. Lighter planets are more difficult to constrain,
especially water-rich versus water-poor worlds. Studying Earth as an exoplanet,
we find a $\sim \pm5$ point "error floor" in iron-mass fraction and $\sim \pm7$
in core-mass fraction from our lack of knowledge on mineralogy. The results
presented here can quickly guide observing strategies to maximize insights into
small exoplanet compositions while avoiding over-observing.
