  Optimal working points or "sweet spots" have arisen as an important tool for
mitigating charge noise in quantum dot logical spin qubits. The exchange-only
qubit provides an ideal system for studying this effect because $Z$ rotations
are performed directly at the sweet spot, while $X$ rotations are not. Here for
the first time we quantify the ability of the sweet spot to mitigate charge
noise by treating $X$ and $Z$ rotations on an equal footing. Specifically, we
optimize $X$ rotations and determine an upper bound on their fidelity. We find
that sweet spots offer a fidelity improvement factor of at least 20 for typical
GaAs devices, and more for Si devices.
