For me when mixing an ambient/sparse track I like to do A/B comparisons. Actually the A/B comparison method applies to almost any kind of mastering/mixing. You don't want your audio super loud compared to the average game but you also don't want it super low. My goal has always been to have the user be able to go from game X or movie Y to my game and not have to change the volume knob too much. The problem is you often face the loudness wars - where everything louder is automatically better.

So shoot for that. Another important element is how well does the music content fit in with any in-game sound effects the project is using. Does the music sit in the background and allow for the sound effects to ring through? The overall audio level (music, sound effects, any possible voice overs) need to be at a level that I can change media back and forth and not have drastic, extreme, super changes in playback level.

Another tip: Do NOT use a normalizing plugin when mixing really ambient/sparse stuff. Or perhaps I should say... if you do then use it sparingly. Instead I would do things by hand which shouldn't take too much time. Since you're going for nuance - many generic normalizing plugins can blow things way out of proportion and ruin that understated feel you're going for in the music.

Finally - consider yourself lucky. It's rare nowadays that I get to actually implement my audio into a game! Instead I often throw it over the fence (so to speak) and the team implements it since I'm often remote.