"Reset to Default" UI for Computed / Synced Fields

Background

Here is a nearly identical question to What's the best "Reset to Default" UI element? but with one significant difference: various fields that may be customized, but in their default state are dynamically generated based on other fields.

For example, assume I have a form to auto-generate sales lead emails:

As noted, on the previous question, resetting the default was best represented as a button because it is a momentary command: fire and forget, "press here and a different field gets updated".

But in this case, the default values are dynamic and being linked to them can be seen as stateful -- when "Use Default" is enabled, the Subject and Body fields will be automatically updated each time a backing fields such as the Contact Name are changed.

Our current UX design allows users to interact and customize these fields, which automatically clears the checkbox. If a user re-checks the "Use Default" option, the text is replaced with the auto-computed value.

Question:

Is this an appropriate UI? Most UX guides indicate that checkboxes generally they should not have immediate effect, and perhaps a slider is the better option? Is visual feedback that the fields are in "computed" vs "custom" state necessary, or is there a better alternative?