What is the target audio level range for the final mix?
This is another "how long is a piece of string?" type of question. Unless you are mixing to broadcast specifications there are no rules or guidelines.
It's a quiet ultra low budget indie with very subtle sound design..... Should I just push it as far as it can go without distorting?
Well, if it's a "quiet" film should the dialog be as aloud as the score in an action film? Should every door-knock be as loud as a gunshot in a war film?
The only way I can get a sense for an acceptable level is by mixing down and testing it next to another film but there has to be an easier way.
Sure; the easier way is to convert your tracks to Pro Tools, and take them to a full size, Dolby certified mix facility and do the rerecording there. That way you are mixing in a theater similar to the ones in which your project will be screening. That's the easy way; it's also very expensive.
Otherwise you have to do just that; mix and compare to a DVD of a similar film - it's not going to do you much good to compare your quiet film to the mix of "Iron Man 3."
Mixing a feature film without the proper tools and without the proper environment is a really, really tough proposition. I've done exactly the process I described above almost endlessly. I've watched all sorts of films over and over again in my studio - which is isolated and decently treated, but is NOT in any way comparable to a proper mix stage - and the same films in my living room and in a friends living room. When I first got started and was readying to mix a feature or other important project I would watch two or three similar films in all three places several times and try to approximate. I couldn't really approximate, of course, as the project on which I was working was different than the ones I was comparing it to, but at least I had a baseline for comparison. Then I would mix, burn a DVD, and audition the mix, listen to one of the similar films; go back, tweak the mix, burn a DVD, etc., etc. - repeat as needed. The second mix/burn was usually the one as I got more experienced and became thoroughly familiar with my room, but it took me half a dozen or more mixes back when I got started.
I no longer do the round-robin listening sessions. My mixes translate fairly well these days, but are not even in the same league as when I get that rare opportunity to have a project mixed in a proper facility.
I'm sure that APE will chime in.....