The 85th percentile is the cutoff time 
such that
In other words, the 85th percentile refers to the time needed to belong to the top 15% of the distribution; more generally, the
percentile is the top 
of the distribution.
Anyway, to find this value of, transform 
to a random variable 
with the standard normal distribution using
where
is the mean of and 
is the standard deviation of .
Here
is used to denote the z-score corresponding to the cutoff time . Referring to a z-score table, you find that this occurs for 
. So,
In other words, the 85th percentile refers to the time needed to belong to the top 15% of the distribution; more generally, the
Anyway, to find this value of
where
Here
