A regular octagon consists of 8 interior and 8 exterior angles of which the respective sides and angles are equal. The exterior angles total 360 deg. So 360/8 = 45 and if you are cutting wood to form a 45 deg angle each cut must be 1/2 or 22.5 degrees. This will be added or subtracted from each side of the stock. Your table saw normally is set at 90 degrees, or perpendicular to the top. Subtract the 22.5 degrees from 90, and you get 67.5 degrees.
Answer: Set your blade tilt to 67.5 degrees.