You can try this:

CREATE TABLE #Analysis

(

ID INT IDENTITY,

Margin [numeric](21, 6) NULL,

Gallons INT,

Freight [numeric](21, 6) NULL,

AccMargin [numeric](21, 6) NULL)

INSERT INTO #Analysis ( Margin,Gallons,Freight)

SELECT 0.050220,5022,-30.180000

INSERT INTO #Analysis ( Margin,Gallons,Freight)

SELECT 0.050220,-5022,318.260000

UPDATE #Analysis

SET AccMargin = FLOOR(10000000*(CAST(MArgin AS DECIMAL(22,7))/CAST(Gallons AS DECIMAL(22,7)) +

CAST(Freight AS DECIMAL(22,7))/CAST(Gallons AS DECIMAL(22,7))))/10000000

SELECT * FROM #Analysis

DROP TABLE #Analysis

You might also want to try it without the CASTs but I think they'll be necessary.