usability – How to represent an unpopulated initial value?

If the value is populated, set the expectations of your users so that they see this value. Let them know that the system is working to return the value …

Give the user the feeling of control (system status)

At first glance, the use of – gives the value a zero or unimportant appearance.
If the application will certainly provide a value, be it between 0% and 100%, it is essential to show them the final result. system state.

Jakob Nielsen points out Visibility of the system state:

The system should always keep users informed of what is happening, through an appropriate return of information within a reasonable time.

How to view status

Here you have a choice. You could:

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NOTE:
I did not mention a percent loading bar, since your final value is a percentage itself.