When placing/mixing Sound Effects, and the actors on screen are outside, I always seem to have a little trouble finding the right reverb. with all sorts of rooms and chambers, I pretty fast hear in my head what I'm looking for and getting the verb I want goes relatively fast. But with those outside scenes I find myself fiddling the controls without a real starting point.
My best luck has been with impulse verbs. Find a good impulse as a starting point then tweak whatever variables you can to lock it down well. I just did this yesterday for some sounds occurring in a small town area. Came out pretty well.