If anyone cares, the chance out of 400 (rolling 2d20 yields 400 possibilities) of getting a 20 with advantage (or 1 with disadvantage) is 39/400. It then decreases by 2 for each number lower (or higher) until you get to 1/400 chance at 1 (or 20). So 37/400 for a 19 with advantage, and so on...