how does eye prescription math work?!

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If someone needs -8.00 glasses to see 20/20, but they are wearing -4.00 glasses, they will be like a person who needs -4.00 glasses (but is not wearing them). I can tell you their vision will be a LOT worse than 20/40. I'm thinking they would need somewhere in the neighborhood of -7.50 glasses to see 20/40, but it is difficult to say, because the visual acuity numbers and glasses prescription numbers are two totally different parameters and don't perfectly relate to each other. (It is not like converting degrees C to degrees F).

My glasses are about -4.50 and, without them I am not even close to seeing the top letter on the eye chart.