I was an Optometry Tech back when I was in the Air Force. Once I had a pilot in the chair and asked him to read the bottom line. He started to call off letters that was not on the chart. Or so I thought. He was reading off the Americal Optical logo from the bottom of the chart. I had to walk down the lane to see what he was reading.
To answer the original question, normally the bottom line on an eye chart is 20/10.
Bottom line is 20/10. However, as you get older you should be able to read the bottom lines better, as you lose your ability to accomodate. That is why old people get reading glasses, so that they can read without holding the stuff at arm’s length, or on the floor.
Driver’s tests are horrible for testing however. My ex has a problem with the eye muscles so that she can only use one eye at a time (she had surgery when she was 6 mos old, and doesn’t know what the problem was, neither do her parents). So her depth perception is not spectacular. She can pass the test with flying colors because they don’t do each eye individually. I had one test me and said “20/15”, so I asked if I should take my glasses off. “You mean you have glasses?”. I was wearing them.
I’ve always had eyes like a hawk (little beady ones in the side of my head… no, wait…) I remember as a kid laughing at a Daffy Duck cartoon in which he did the “Eye chart copyright etc” last line gag. When I went for an eye test, I found that not only could I do that for real, but the tiny “printed by…” font was probably up to five times larger than it would have had to be for me to have any difficulty.
Now, at 34, my vision has worsened. It’s only extremely good now. Still better than most people’s, but as a kid I was able to freak people out it was so good.
Which is a bugger, coz glasses would suit me. 
When I was younger I could read the bottom line just fine with both eyes. Age has change that, but, at 58, I still have 20/20 distance vision. My near vision has not faired so well.