Question: I want my marker to appear not in the center of the screen, but 25% of the way up to give extra room for the popup box. Although sticking an offset in is easy, the offset depends on the zoom level as if you're zoomed far out, you'll want to center the map quite far up (such as 50km). If you're really zoomed in, then you'll want to center it just a tiny amount like 10 meters.

I'm not sure how to accomplish this. Any ideas?

Answer: 
Try this: 
Take the height of the plugin and get 25% of that. 
Then you need to multiply that by the degrees or kilometres per pixel scale at that height (if you can't get it straight from the plugin then I guess do the math), then centre the screen at that point on the globe.
I am not sure as it has been a while since I messed around with the API, but I think you can create a custom marker object with its own pixel based offset, and display that instead of the default marker.

EDIT: Whoops read it again and realized you probably just wanted to move the whole map and not the marker.