Dried extracts adds roughly 45 total gravity units. To find out exactly how much you would need to add you can do this simple equation...
(Total gravity target - Total gravity from mash)/Extract per pound GU value
Total gravity is total volume x gravity. So I will assume you have 5 gallons....
target is 5 x 49 = 245
From mash is 5 x 42 = 210
Extract value is 45
(245-210)/45=.777 pounds of dried extra which is 12.5 ounces so you were right around where you needed to be.
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