I find Sean Terrill's calculations extremely accurate for FG. I downloaded his spreadsheet, but it looks like brewer's friend is also using his calculations (link posted by boydster above). I use his default wort correction factor of 1.04, though on the brewer's friend site they discuss how to set that for yourself. I've checked a hyrdrometer against his new cubic calculations at last count on 25 batches: 19 were within 1 gravity pt or less, 5 within 2 gravity pts or less, the only one farther off than that was a saison that went down to 1.002. I just rely on it now for the great majority of my small batches.