Double tap centers the view. That is understood.
But I’m confused because it also sets the zoom. And I don’t understand what this zoom is based on?
Sometimes it zooms in, sometimes it zooms out, until the zoom is fixed. I didn’t get the trick yet.
The double tap also seems not to respond sometimes. It seems volunteers but I didn’t get it.
If you could light me... thank you 
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On iPad or iPhone
When you looks straight ahead and change the zoom and leave that setting for a while it becomes your new personal preset. When you look around and double tap the screen it resets straight ahead and to that last used FOV.
Be sure to hit an empty space in the cockpit. The lower EICAS in the B777 has a clickspot, double tapping that doesn't do anything because it's a clickspot.
Thank you this explains the variation of zooms I could see. 
