The 90th percentile tells you the value for which 90% of 
the data points are smaller and 10% are bigger. 
Statistically, to calculate the 90th percentile value:
1. Sort the transaction instances by their value.
2. Remove the top 10% instances.
3. The highest value left is the 90th percentile. 

Example:
There are ten instances of transaction "t1" with the values 
1,3,2,4,5,20,7,8,9,6 (in sec).
1. Sort by value -- 1,2,3,4,5,6,7,8,9,20.
2. Remove top 10 % -- remove the value "20."
3. The highest value left is the 90th percentile -- 9 is 
the 90th percentile value.