How bad do your eyes have to be for you to wear 6.0 glasses?

So I'm curious how bad your eyes have to be for you to wear 6.0 diopetres in your glasses. I wear 2.50 in my left eye and 2.75 in my right eye, and I struggle to see 3 metres in front of me without it being blurry. I'm also asking this as I have a really important eye test coming up soon, and I need to pass it, and obviously before I go to the test I wanted to ask on here to see if anybody could settle my nerves a little bit. People with 6.0 diopetres, do they struggle to see there hand at arms length without it being blurry? I can see my hand perfectly at arms length. As I said, I really, really need to pass this test. It affects my future if I don't, quite heavily.
Added (1). Also I don't have any problems with my eyes like glaucoma. I have to wear glasses full time.
Added (2). Also, how strong to your lenses have to be for you to be classes as legally blind?
Added (3). For me to be prescribed 6.0 glasses, on the Snellen Eye Chart, which letter must I be able to see? I can see the second to third row down on the chart when in the opticians.

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