Attempts to model dynamics at these long timescales using a second stretched exponential or mono-exponential function were unsuccessful. Whilst including this data in the fitting process does not negatively impact the fitting of Eq. 11 due to the small moment at long timescales, it does not add anything to the model. Therefore, we repeated the full fitting process with datasets trimmed to only include M(t) ≥ 0.01M 0 (Fig. ), which gives equivalent parameters to fitting the whole decay trace. To investigate the impact of fitting incomplete decays, as may be required for very slow relaxing samples, we trimmed the decay data at 0.05M 0 and 0.1-0.9M 0 in steps of 0.1 (i.e. 10-90 and 95% of the decay included), and repeated the fits with M eq fixed to zero or freely fitted (for this test t offset was fixed to zero and M 0 fixed to the first measured point at target field). We find that allow-ing M eq to refine freely quickly results in unphysically-large values of M eq , and diverging values of τ * and β as less of the decay is included (Fig. -S45, Table and Table ). In contrast, fixing M eq results in much more stable fits 32 (Fig. and Fig. ), and is absolutely necessary when fitting incomplete decays. We recommend measuring the decay to 0.01M 0 and fitting the trace with M eq fixed; minimally, measuring to 0.1M 0 and fitting with fixed M eq is associated with an acceptable error of 5% in both τ * and β .