Analysis of the empirical uncertainty for short term repeat observations of RV using filter HR10 and HR21. Plots a) and d) show the variation of ERV with vsini. Red lines show the curves predicted using the model value of C (see Eq. (A.4)). The dashed lines shows the curves predicted using values of B and C fitted to the binned data. Plots b) and e) show the variation in the estimated value of parameter B in Eq. (1) with S/N. Red lines show values of B fitted to the full dataset for each filter. Blue crosses show the estimated values B as a function of S/N corrected for the measured variation of B with Teff (see Table 3). Plots c and f show the normalised uncertainty for short term repeats and long term repeats. The black curve shows the CDF for short-term repeats. The red line shows the CDF for long-term repeats. Blue diamonds show a Student’s t-distribution with ν = 6, which matches the distribution for short-term repeats well.