.
First,
percent here is an adjective, so that number is irrelevant:
0.01 percent alcohol, 50 percent alcohol.
Second, there are only two chances in total: (1) it happens and (2) it doesn't happen. The sentence speaks of the probability of
one of these chances only, to wit: it happens.
Therefore:
A 0.01 percent chance of it happening, a 100 percent chance of it happening..