I was curious about this so I hit the 'Log A Call Button' in my dev org and ran a SOQL query to see if any fields had data to tell us it was a call other than the Subject line and found the TaskSubtype field had been updated by the system with the value of 'call'. I checked in reports and it is a field you can filter on.
All fields that are "number" fields in Salesforce is always displayed in the user's locale, including currency fields. To control the display, change your Locale in the Personal Settings area of Salesforce.
I'm not sure if you can create this formula field (I tried it but didn't suceed). What I did instead was create such formula directly on a report.
Create a Custom Report of Leads or edit an existing one.
Click on Edit in the Fields Available for Reports section
Click on Add fields related via lookup » on the right.
Find Converted Account and add your ...
After some trial and error, I found that changing the report to a matrix report and grouping rows by stage, columns by Close date and using the PREVGROUPVAL to show the summary at both Stage and Close Date allowed me to display data based on PREVGROUPVAL for each stage.
If I understood well the scenario, there is a specific feature in Marketing Cloud that may help you to build and track these control groups vs the rest of the audience. It is called A/B Testing and it works similarly like the approach you shared using Split Activities in Journey Builder.
I found it. Took 3 days and a good amount of luck.
Basically, you have to add a new community page called "Report Editor". It's a standard page but using the "Build Your Own" template, it's not actually there until you add it.
Once you've done this and published the community... the "New Report" button shows up on the list view for reports. Done.