If your sale and reserve numbers don't need to be aggregated (i.e. although you might use Sum in your chart you are only really adding up one number) then you can use if, so something similar to;
Sum(if(sale>0.6*reserve,sale,null()))
(You may also need to turn off Include Zero values/similar)
If you did need to add up sale & reserve numbers before doing the test then it would be a bit more complicated, but is possible, would depend on your data model.