I think you need advanced aggregation here to embed your Sum() aggregations into another Sum() aggregation.
Look into advanced aggregation examples into the help, like 'Sum of rows in pivot tables' (even if you are not using a pivot table).
You are not showing full information about your chart, like the dimensions used (it's in general much easier to help if you post a small sample qvw). I am assuming a single dimension called Dim1. Then your expression might look like
if((Sum (met_core_sl))/(Sum (answered_call))>.8,
Sum(total aggr( if((Sum (met_core_sl)/Sum (answered_call))>.8, 1,0) , Dim1 )) /18