I hear you, I always fear changing things when its running smoothly, then you wonder if you did the right thing! That's great that you've been bf that long, I had to wean my first to go back to work as well.
I had a thought about the evening feed (just before bed I'm assuming?) Why not try giving her a bottle to see how much she actually takes, then afterward you could let her bf just for the few minutes of comfort. That way you could find out how hungry she really is at that time.
My LO takes 6 oz now, but if I remember right, my oldest used to take 8-10 oz by the time she was that old. She like a particular bottle too, so I just made up 2, one with 6 oz and one with a few more ounces that way we didn't have to switch bottle types. It did make more dishes, but she soon didn't need as much formula b/c she was eating more solids.
I hope this helps,
Heidi