Here is a link to a paper that reported the results of experiments to estimate the relationship between visual acuity and refractive state.

http://journals.lww.com/optvissci/Abstract/1990/01000/Effects_of_Dioptric_Blur_on_Snellen_and_Grating.2.aspxI determined that Figure 1 in the paper can be approximately summarized by the following equations:

1) A = 20 when D=0

2) A = -48 - 88*D when D is less than or equal to -1

where A is the denominator of the visual-acuity fraction (e.g., in 20/40, A=40)

and D is the refractive state measured in diopters (a negative D indicates myopia).

Equation 2 generates the following:

D=-1 -> 20/40

D=-2 -> 20/128

D=-3 -> 20/216

D=-4 -> 20/304

Let's solve equation 2 for D in terms of A:

3) D = -(A + 48)/88 when A is greater than or equal to 40

Equation 3 generates the following:

20/40 -> D=-1.0

20/60 -> D=-1.2

20/80 -> D=-1.5

20/100 -> D=-1.7

20/200 -> D=-2.8

20/300 -> D=-4.0

We would expect someone with the visual acuity to the left of the arrow to need a lens with the diopters listed to the right of the arrow in order to clear the 20/20 line. For example, we would expect someone with 20/80 visual acuity to need a lens of -1.5 diopters in order to clear the 20/20 line.