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.