If we tried to force a 1600x1200 cover photo into an iPhone portrait mode screen that only has a 640x1136 resolution, the photo would be extremely distorted. Instead, we have to maintain the aspect ratio and center the cover photo around the focal point while still filling up all of the space on the screen. So there isn't really a standard ratio as it really depends on the screen size/device etc. The focal point is a great tool to use to ensure your cover photo will display perfectly on all devices including mobile
What is a cover photo and how do I change it?
Cover Photo Focal Point.