For purposes of stronghold prestige, how is stronghold completion calculated? For example, let's say I have a starship space. I can put one starship-sized deco there, or a ton of small decos. Do I get full completion points for the space with one large deco? Or is it better to subdivide the space and fill it with small decos?

It's based on the number of decorations placed versus the decoration limit, I believe. Once you get to 100% of the limit, you can't place more decorations even if there are open hooks, and I believe that all strongholds can go to 100% without subdividing hooks.

 

And, of course, the decorations-placed limit has two versions:

1. How many you are allowed with the current open rooms.

2. How many you are allowed with all rooms open.

 

The percent-complete (and consequently prestige calculation) depend on the second, not the first, so you must open all the rooms if you want to reach 100% decoration-placement.

 

Note that the above is for prestige. The stronghold bonus for Conquest is based solely on the number of open rooms.