Hi there,
I came here to post these 2 things, but I see that they are already being mentioned So here's my 2 cents:
- Counting down makes more sense imo, 1 less calculation you have to make yourself.
- Having a unit behind a tall building is a problem. I like the mouse-over transparency idea, maybe that could work for a selected unit behind a building as well. But I don't know if this is hard to implement, and it could look weird. You could also use a silhouette for a soldier who is behind a tall building, so you don't actually see the soldier but you can see where he is.