An ideal magnetisation decay measurement captures the magnetic moment of a sample as a function of time, M(t), from when the saturating field is instantaneously switched to the target field until the moment has reached its equilibrium value. The magnetic field reported by a superconducting magnet system can deviate from the real magnetic field in the sample chamber by tens of Oe, especially at low fields, due to pinned magnetic flux lines and flux movement. Known techniques to reduce the remanence of the magnet (oscillating to the target field, resetting the magnet or using the ultra low field accessory) are incompatible with the premise of the magnetisation decay measurement. The remanent properties of the magnet are reproducible for a given field charging sequence, so the magnet can be calibrated with a standard sample for a particular experiment, and the target field set to achieve the desired real field. Precisely zero magnetic field is essential if relaxation rates are highly sensitive to applied fields, which is the case for single-molecule magnets, and allows M eq to be independently defined and fixed: zero in zero field, or accurately predicted for specific temperatures and fields using an experimentally-calibrated model Hamiltonian.