Log event rate

What do you mean by events per second? And why is this value important to you? 

For SIEM solutions scoping. Sentinel for one just need digest values total per day.

In the Fortinet space the Fortianalyzer gets licensed this way.

In the Fortinet space the Fortianalyzer gets licensed this way.

There is something in SFOS: https://www.sophos.com/en-us/products/next-gen-firewall/central-reporting/sizing-tool

But this is just a sizing tool. It highly depends on your Environment. 

Need to figure out how much we're currently producing, actual values on an in production firewall.

As far as i know, this is not logged. You could integrate the firewall to Central and check for the time span of 7 days, how much data is stored. 

Sounds good, where in Central does one find that information?