[rrd-users] average of 95th through 80th percentiles, in a sense

[Ryan goldberg]

Our upstream does 95th percentile billing in a weird way.  Quote:

"Blahblah Telecom monitoring system takes and records a sample every five 
minutes for both the in and out going data stream.  At the end of the month, 
the samples are ranked and the top 5% of the samples are excluded and the next 
highest 15% recorded samples are used to compute the monthly sustained usage. 
  If the monthly average is larger than the base commitment level, the base 
commitment level is subtracted from the monthly average.  The result is used 
to bill the burstable usage.  The burstable useage rate is $blah/mps"

Can someone point me in the right direction so that can I produce this number 
with rrdtool?  I have 60 days of 5 minute samples...

TIA-
Ryan