Two overlapping lines are required to provide results. As with all patch testing that involves spatial depth change, results depend on accurate positioning. Under typical conditions, latency test results will vary by about a half a second, which may be more than the actual latency. So once again, my recommendation is to track the history and use the average results over many tests to remove the errors. At lower speeds it can be difficult to stay on the survey line and at higher speeds the sounding quality may degrade.
Most multibeam surveyors use trial and error to find the happy medium.
It is also important to track squat change and to adjust for it if the vertical offset is significant. It is a GPS offset.