I have read different threads about wind speed and I've gathered that below 10mph is a good starting point for smooth video and all around easier flying. However, when I look at the UAV Forecast app on my phone, I see that I can change the height at which the wind speed is collected. The default is around 30 feet. At 400 feet, the speed has doubled. What is everyone referring to when they say 10mph? 30ft high? 200ft high?

As you have no doubt discovered wind speed does depend on the altitude where it's measured.  Trees , building and other "ground clutter" tend to block a lot of the wind around the surface (the 30 foot mark is most likely what they determine as the average height of trees)  If most of your flying is going to be at the upper altitudes, set your app to that range.  It's a quick, handy-dandy reference you can use for a quick idea of the flying weather but just a reminder, it's only for planning purposes so you can be aware of the most obvious risks like high wind.  it's not likely to be any more accurate that the weather report you might on your local news channel.  

I have my UAV app set to 164' (50 meters) that's my RTH height. That's what I use because it I'm far out and need to get back on a windy day, I want to know I'll make it back. So if I'm flying at 300 or 400 feet and having issues with the wind, I know going down to 50 meters I should be good to get back and clear all trees or hills.

Sounds good! So what wind speed do you guys prefer?