I use 81% as the VF for my generic RG-6. When using the VNA to locate a fault even at 300 feet down the line I find the damage within a couple of feet of the VNA called out distance. (working farm and steel blades pulled behind a 20,000 pound tractor don't even notice RG-6 when someone forgets to pick up the tool at the edge of the field) Now, there are lots of opinions on this topic. Some seem more based on hearsay as opposed to simple testing. As far as fancy testing methods I have always found that a drum of coax on my bench and the use of an oscilloscope to measure voltage at both ends with a 5 watt, or so, input signal gives me good enough results for the real world. The NanoVNA has made that type of quick check much faster and easier.