That's way too low for a comfortable installation. Any possibility of raising the flange? Do you have access from below?
If you can't raise it, consider some extender flange rings. A proper installation is on TOP of the finished floor, so you have a lot of ways to go. It'll take a few rings and some long screws to hold it together along with some silicon caulk. As to the space required, Terry's discussed it before. I think it requires 11.25", but a quick search should tell you as should a look at the spec sheet drawing.