Yes, as John says let you database generate this number, if you need to use the number in the front end then just have your procedure return it after the insert has completed.
Something like...
CREATE PROCEDURE example_procedure
(
@SomeVariable INT,
@AnoherVariable INT,
@UniqueFieldID INT OUTPUT
)
AS
SET NOCOUNT ON
INSERT INTO
TheTable (Field1, Field2)
VALUES
(@SomeVariable, @AnoherVariable)
SET @UniqueFieldID = SCOPE_IDENTITY()
RETURN @@ERROR
SET NOCOUNT OFF
GO