Max HR and "zones"

From what I have seen it seems that as a 21 year old female my maximum HR is just under 200. I tend to find myself comfortable at above 85% of my max HR for 40 minutes without much trouble...in fact unless I am hitting 85% (ie 170bpm) I don't feel like I am working hard. I am happy to go up to about 190 before I start to feel like I am straining and might need to slow down at some point.
I am generally in good shape in that I do a lot of physical stuff, loads of walking every day and am quite active aside from my exercise, but I am concerned that maybe I am not working efficiently for weight loss in my exercise perhaps because of my general fitness level being ok.

Can someone please help me get an idea of what a good HR is for weight loss NOT muscle gain (as I also do gym ball stuff for that and it's not really a look I am going for), and tell me if what I am doing is ok because it doesn't seem to add up with what I have read about zones and stuff.