As with so many other things, "it depends" is the answer. When driving in areas with good supercharger coverage or other reliable charging options, 30 miles is an ample buffer. But something that happened to us a few months ago creates the "it depends" scenario.
We have a house in the Sea Ranch and usually can get there from our home with about 80 or so miles of range left. We had to make a bit of a detour on our last trip (about 25 miles each way on the way to Sea Ranch) but figured that arriving with 30 or so miles range would present no problem.
As it happened, we had a pretty good coastal storm north of Jenner on Highway 1 and about 10 miles from our house the CHP had to shut down 1 due to some trees that fell across the road, which resulted in a fatal accident. The CHP officer told us it could be many hours before the road was open again and he suggested we head back south on 1 to find a side road that would get as passed the closure area.
Unfortunately, the Tesla maps/nav didn't have any internet connectivity in this rural area so we had to take a guess as to which of the side roads would lead us around the closure. I was lucky in that I had previously downloaded a map to my phone for use when I'm deer hunting in the area, so I could see some basic road info, but the map package I had (Backcountry Navigator with forest service maps) didn't have the kind of road detail needed to know for sure which roads led where for the area we were in.
Ultimately, we had to drive 10 miles south, take a side road that led up to to top of the coastal ridge, drive along that road north for about 15 miles and then re-connect with Highway 1 passed where we thought the closure was. That connected us back to Highway 1 about 5 miles from our house. The problem is that we used up a LOT of range driving up what was in essence a one way dirt road and only got a fraction of that range back descending back to Highway 1. We ended up at our house with about 2 miles of range left.
If we didn't have the saved map on my phone, the odds are that we would have had to drive blindly to find the right side road and we definitely would have run out of range in an area that had no charging, let alone phone service.
So the short story is, it depends. Our story is something that will likely never happen to most people. But when you are in a rural area, 30 miles of range is, in my mind, not a safe buffer. (By the way, on muddy, steep, dirt trails, the P85D actually is fairly adept as an off road vehicle).
