My First Stop at a Supercharger

I've had my Tesla just shy of three months, and had my first stop at a supercharger last night. When I bought the car, I made sure to get a model that stored enough energy I could make it to my parents' house and back on one charge, they live about 72 miles away. My 100% range is in the neighborhood of 240 miles, so I have plenty of miles to spare. This was not my first time making this trip, and I have not had an issue before. But this time my siblings were all in town, and two of them wanted to drive the car. So I let them, and we took it out on the highway so they could try out the autopilot. The extra miles of driving we put on the car were enough to put me out of the comfort zone for getting home. I recall that I worked out that as long as I have 38% charge left in the car, I could make it to my parents' house in case of emergency, so I routinely set my charge schedule so that even at the end of a day of regular driving I still have 40% charge left in the car so I could make an emergency trip without spending time charging if I got a call. But after the test drives, I was down to 34% charge. I hit the navigation button for home, and it directed me to stop by the supercharger in Charlottesville on my way home, and charge for 15 minutes. But then it said I would arrive home with 28% charge in my battery. That seemed useless, why so much charge? I might have been able to make it all the way home without stopping if I kept my speed down, but I didn't much feel like doing 55 on the highway when the speed limit was 70. And the supercharger was maybe just 1/4 mile out of our way, this was not a big deal. So when I arrived at the supercharger I had about 8% left in my battery, and it said my range was something like 21 miles. Now, I only live about 12 miles from the supercharger, so maybe I could've made it home just fine without stopping, but having no experience in how the car performs with a low charge, I decided to play it safe and get a little extra charge. I plugged in, and it was charging very fast since we were the only car there. The kids were sleeping in the back seat (it was 10pm) and my husband and I just sat and watched the numbers, and just charged it up to 14%. It didn't take long at all, although perhaps a bit longer than if we had pumped gas traditionally (and we would have gotten a full tank, not just a little top-off). The light on the dashboard had turned yellow somewhere around 45 miles of range, maybe? I wasn't paying too much attention, but I noticed at some point the battery symbol had gone from green to yellow. By the time I got home, I was back down to 8 or 9%, and I had expected it to turn from yellow to red at some point, but it didn't happen below 10%. Maybe it would have happened if I got below 5%, I don't know. It might be something that is discussed in the owner's manual, but I don't remember it. Regardless, it was a reasonable first experience at a supercharger, and now I know exactly where the charging station is in town. It's useful if I'm coming home from a long trip and need a little extra to get the rest of the way home, although only if I'm returning from the north or the east. When I travel west, there are no superchargers between my destinations and my home, so if I ever run out of power out there, I'll be using the "drive slowly and hope for the best" technique to get home.