From what I understand, the reason for the more advanced calculation post fermentation is due to the alcohol present in the solution. I only use a refractometer for both pre and post fermentation, but I also use the onebeer online conversion or beersmith's conversion for post fermentation calculation.
Here is a link to onebeer -->
http://onebeer.net/refractometer.shtml
Note, that you need both the OB and FB (original Brix and Final Brix) for the calculator to work properly. I've found this to be close enough for my needs.