Trying to answer my own question.
I don't have access to the real northern sky, but I have done an experiment by using a tablet as the sky by opening SkySafari on it and let my mount takes images of the screen 

I blocked the screen for like around 40% and EKOS platesolve works effectively. I've pushed it and found if I covered more than half of the FOV then the platesolve will fail.
So if the FOV not block more than 40% I think it's fine.
 

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