Select row at the nth percentile

Question

I have two tables, table1 and table2. Let the two tables contain date, id and latency column.

I have a simple query that performs a join on the two tables and returns a set of rows:

Select table1.date,(table2.latency - table1.latency) as ans from table1, table2
where table1.id = table2.id order by ans;

I need to find the nth percentile row from the returned set of rows, lets say I need to find 90%, 99% and 99.9% percentile row from the data.

I need to display the data in a form like this:

    date       |   percentile  | ans
    01-12-1995 |    90         | 0.001563
    02-12-1999 |    99         | 0.0015
    05-12-2000 |    99.9       | 0.012

This is my first encounter to PostgreSQL. I am confused as to how should I proceed.

I was having a look at PERCENT_RANK() function. Please guide me in the correct direction.