Entity's viewFrom option in Cesiumjs

I would like the camera to keep tracking one of my entities by setting viewer.trackedEntity .
The problem is that in order to control the camera position when I start tracking my entity ,
I need to set entity.viewFrom .
viewFrom expects a Cartesian3 in East-North-Up coordinates relative to my entity’s position.

What I could not figure out is :
Given my former camera position and my entity’s position, how do I calculate viewFrom so
I could keep my former camera position after setting viewer.trackedEntity ?

Reference to a former topic I read on the subject:

Do you mean like this? Select a spot on the globe, click on ‘All Passes’ for the first object (Vanguard 1), then click on a pass, then animate.

Yes its exactly it

Use Camera.setView on every tick, updated with heading/pitch/range of the object you want to look at in the local reference frame.

Here’s an example of setting the view statically.