I have an RXII and it took awhile for me to get set properly for me. I have mine in rifle mode I believe and it seems to be doing a good job. If you hold the button down while ranging, it will "scan". Sometimes it will pick up the little branches that get in your way so use common sense while using it too. Im not sure if I have mine set on first or last return but it does have a setting for ranging through brush/fog. I believe thats the setting I had to change. Once I got it set, I left it alone so im sorry I cant be of moe help.
I normaly get set in my stand or blind and then start ranging trees or terain features. Only a few times have I ranged deer and when I did, it worked. The farthest I have ranged anythign was 700 yards I beieve and that was an old car.