I'm trying to sum up the top 80% by value of sales $ and give a count on how many customers make up that 80% : I'm trying to build a sales query for a rolling 12 months.  I know my sales by customer, and my total sales.  I'm trying to sum up the top 80% of sales $ and give a count on how many customers make up that 80%.  Any ideas?  I have a result set that looks like the below. Thanks in advance!

Customer	Sales	    TotalSales	    PercentOfSales
8585	    19788.81	769658.68	    0.03
8429	    19598.26	769658.68	    0.03
2837	    19431.29	769658.68	    0.03
6071	    19398.11	769658.68	    0.03
5027	    19223.13	769658.68	    0.02
6677	    19204.90	769658.68	    0.02