I made conversion chart that shows FG values for different OG"s, with percentage of yeast attenuation.
That was something that annoyed me when I started to work with refractometer, but not anymore..
It cover values from 15 Brix (1.061 OG) to 11.8 Brix (1.048 OG) with related attenuation values from ~45% to ~80%.
Example how to use it: we have wort with 12.8 Brix starting gravity (1.052) and at the end of fermentation it is at 7.2 Brix. Read value that crosses between 12.8 in column and 7.2 in row- 1.015 with 71% attenuation.
Since Excel adds weird bold lines at pdf documents I uploaded .xlsx and Google docs version.