Allow Calculated Metric as Part of Other Calculated Metrics

It would be really great if choosing an existing calculated metric was an option for creating another calculated metric.  For example, create a "total acquisitions" calculated  metric that sums 4 different success events. 

But now I want to create a cost per acquisition, or total acquisition conversion rate - I now need to add these 4 success events again into this new calculated metric.  Plus, if new success events need to be added to the total acquisitions, I'd now have to fix all 3 calculated metrics instead of one.  It would just be a lot easier to choose the "total acquisitions" metric as part of the other 2 .