Considering that you're only working with 6 digits to begin with, consider transposing your formula for a smidgen more accuracy.
UPDATE dbo.Analysis
SET AccMargin = ((MArgin+Freight)/Gallons)
Of course, since you're doing decimal division, you should as consider expanding the precision and scale of the calculation by doing as Dwain suggested with CAST and then round to the correct number of decimal places.
--Jeff Moden
Change is inevitable... Change for the better is not.