Is there a rule of thumb regarding positioning objects (Z axis) for realistic depth/parallax?

My original painting has tree A close up, and tree B far away near the horizon. 

In After Effects, I will position tree B on the Z axis so it's far away from the camera, then rescale it to match the proportions of my original painting.

Two questions:

1. In the context of my painting, tree B is hundreds of yards away from tree A. How far should I position it on the Z axis? 

2. Can you tell me a math formula that tells me the exact number I need to rescale the object based on how far I positioned it on the Z axis? 

Thank you.