Allow Calculated Metric as Part of Other Calculated Metrics

[#1]

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 .