http://seanterrill.com/2011/04/07/refractometer-fg-results/

Sean is interviewed on Basic Brewing Radio recently:

http://www.basicbrewing.com/index.php?page=radio

He compiled data and adjusted the equation to correct the post-fermentation Brix readings to be closer to the Specific Gravity reading.

I am not claiming to fully understand all of this, but if any of you take such readings, maybe you want to give his new spreadsheet a try.   I put in a few numbers I had on hand and it seems to be pretty accurate.  I will be taking more refractometer readings after fermentation (something I don't normally do), along with hydrometer readings, and see how accurate it seems to be. 

cheers

Don

p.s.  two posts from me in two days, a new personal record.