You have a 2.833% chance of capturing it per ball. Thus, you have at least a 50% chance of catching it within 25 balls and at least a 95% chance of catching it within 105 balls.
An important note would be that at night/in a cave, a Dusk Ball would have a percentage of 5.525% per ball, with the factors you listed. This makes it more effective than an Ultra Ball, and as Mewtwo is found in a cave it would always have that higher chance.
Note that these calculations are for generations 3-4 (based on your tag). In generation 6, it would have a catch rate of 8.124% with your listed conditions.
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