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Don't worry about it, it was your first shot and I gurantee your next one will be a lot easier to do. The dryness was probably more due to the higher temps than to the trimmed fat. I am like Al, I trim all my fat off and place it on the rack above my brisket. Then when I foil it I just toss the fat away. I keep my temps down between 200° and 220°, and the brisket stays plenty moist.