Problem applying a filter to a specific metric in Looker Studio (GA4)

Hello,

I’m working on a report in Looker Studio based on Google Analytics 4 (GA4) data and I want to create a specific metric in a table without impacting the other data or graphs.

Objective:
To display a metric representing the ratio between:

The number of users who have triggered a specific event (for example: ‘casual’, ‘new’, ‘flyby’).
The total number of users (all events combined).
This metric must be independent and display alone in the table, without other metrics or dimensions being affected.

Problem:
Despite several attempts with calculated fields and filters, I can’t get this result.

The filtered metric impacts the whole table instead of remaining isolated.
Or the calculation is not working correctly.
Questions :

How can I configure a metric that calculates this ratio without affecting the other data?
Is there an optimal method for applying a filter or calculation only to this metric in Looker Studio?
Thanks in advance for your help!