The farthest line that you can interpret the letters for half of that line is what your Sbellen acuity is. When I got prescribed my glasses, I could read the 20/15 line. I didn't see it crystalline clearly though, and rarely can someone see that much perfectly clear (btw, that's the -4 lenses that I don't use). So let's say you can read the big E on the chart and it's 20/200. You can read all the way down to the 20/70 line and can sort of guess based on what you see, half the letters on the 20/60 line, but no more. That means you have 20/60 acuity. That's how the eye doctors use it.
You can read the 20/15 line that sounds like some vision improvement! It could also have been some over prescription.
I guess I should thank my eye doc , I dont think he ever really over prescribed me.