Uncertainties of results are affected by how "stable" or reproducible your measurements are, when all your data are analyzed collectively. They are determined through statistical analysis. If you would like to see these equations, you can look up the Standard you are testing to.

What FanTestic software does, is it graphs your measurements (technically speaking: on a Flow vs Induced Pressure, log-log axis) and a bestfit trendline is generated between these multiple points. The closer your measurements are to this trendline, the lower your uncertainties. The further away your measurements are from this trendline, the higher your uncertainties.

To minimize uncertainty, it is best to take more measurement points (both target and baseline points), minimize Baseline/Bias pressures (e.g. by dampening wind, etc), take target measurement points over a larger range of induced pressures, and take measurements on each target point for longer time.