Dividing the sum of an event property by the sum of another event property

I’m trying to come up with a new metic that is the sum of one event property divided by the sum of another property. 

In my example, I want the total sum of visits that have been confirmed / by the number of invoices that have been created. Batch size is the property that contains the number of visits / invoices. But i don't seem to be able to add them to a formula, when I did, it switches to group by not sum. 

Is there a way I could achieve this calculation. So I can see what % of visits have an invoice

Any help greatly appreciated