In Project for the Web, how to de-link the relation between %Complete and Effort Complete value

Hi All, 
I'm looking for a guidance how to delink the relationship between the %Complete and Effort Complete.

The reason behind this is to maintain allow recording from the same interface, a task completion ahead of time. I know, I can create a custom field to capture this, but it comes with a user-experience cost as user will have to switch to another interface "a model-driven app page" to update the actual %Completion. Currently, I'm using MS Teams integrated project Tab to manage the whole thing from Team members' perspective.

Any Idea?