Clarity of purpose in how a task is completed is the benchmark. If, after you finish your user experience flow chart, the flow for any given process feels convoluted, you need to iterate further and get more feedback. Plainly put, a UX diagram is considered effective when it makes sense.
In addition to that, you’ll want to ensure that you’re using the standard diagramming symbols so that everyone looking at the chart can immediately grasp its meaning instead of having to consult a key.