There may be a better way( see Mark's answer above) , or a more correct way to estimate the values you want. All I am doing is interpolating the value from the known percentiles above and below.
Here is an example of finding 99.865 when the 90th and 100th percentiles are known.
Names Default To Here( 1 );
p90 = 68;
p100 = 70;
p99865 = (99.865 - 90) / (100 - 90) * (p100 - p90) + p90;
Jim