Does my spherical correction correspond to my eye chart reading?

At -8.00 you would not see anything on the Snellen eye chart without glasses. The largest letter on a Snellen chart at 20 feet corresponds to 20/400 Snellen acuity (sharpness of vision). Without glasses you would have to be 10 feet from the chart and you may be able to see the largest letter, which at that distance may be around 20/800. There is not an exact translation from amount of myopia (nearsightedness) to Snellen acuity and it will be different for different people. There is a correlation but not 1:1.