The chance is not 2.5%, but instead is a multiplier to the catch rate, a. Here is the formula:
a is the overall catch rate.
HPmax is the maximum health of the Pokemon. In this case, let's say you caught Level 50 Munna. With 0 EVs and 0-31 IVs, this ranges from 136 to 151.5.
HPcurrent is the current health of the Pokemon. Since you use Super Fang, this will be 1/2 of HPmax.
rate is the catch rate of the Pokemon. Munna's is 190.
BONUSball is the Poke Ball's multiplier. The Ultra Ball's is 2.
BONUSstatus is the current status of the Pokemon. Since it is asleep, this value is 2.5.
Plugging this all in, we get a equal 633.3333...
But what does that have to do with anything? Well, for critical captures, a number between 0 and 2047 is generated. If this is less than a times the multiplier (in this case, 2.5), a critical capture will occur.
a times 2.5 is 1583.3333... Therefore, Critical Captures have about a 77.35% chance of occurring under your conditions.
Since you got the captures 16 times in a row, your formula would be (0.7735)^16, resulting in a...
1.641969510729874658525195052896% chance of occurring.
Source
