I had to think about this for a while. The distance and spray setting are co-dependent. So if you spray closer, you’ll need slightly less paint than when you spray farther. I probably gravitate to 8-12 inches, for no other reason than it feels about right most of the time. Sometimes, like spraying into the spare compartment, under the dash, or under the rear scuttle, you have to spray up to 30” away. For that I adjust the pattern more oval or round and dial up the paint adjustment. I also think when I have to lean way over a bonnet to reach the center, I tend to back out to about 14”, mainly so I don’t drag the hose over the bonnet!?! With thin paint I also tend to move away a bit, as that gives better control to prevent runs. Very thick paint...especially when I got caught with too small of a tip...makes you get closer to keep the spray wet, so it doesn’t dry out enroute to the panel.
So my answer would be to use what’s most confortable for you...and then learn to adjust when you need too?!?