It looks like the solution to your question is very easy to look at and calculate. Sort the information by time, take 80% of the total population and then take the time from whatever record number that ends up being. This would give you the time it takes that person to do 80% of their work. The difficulty with Monarch is that it doesn't have alot of functionality to work between records. As far as I know there isn't a way to use totals or sums from other records in a calculated field. You may be able to create a table from the information and then use some of the lookup functions to work your way around it, but I'm not sure how you could get Monarch to calculate it, but I am sure there is a way, usually there is.