7-8 feet is ideal. 10-12 can work, but it depends on the focal length of the camera and the distance you want to ID. If the camera is a higher focal length and the distance is further out, it will help flatten the angle and allow a higher installation height.
I am in a similar situation where my house is 22' soffits on the sides and back and 11' soffits on the front. I ended up moving cameras down and it made a huge difference.
Below is the link to my thread that I usually post whenever someone asks about cam placement, focal length, and height. It shows the difference that can be had by changing mounting height or going with a more appropriate focal length. In some instances I would have thought that going full 12mm on the varifocal would have been life changing, but it wasn't. However, compared to a similarly positioned 2.8mm it was night/day, but until you see what you can see (or not see) with a given focal length it can be hard to visualize what will work for a specific location.
I have gained a lot of knowledge since I signed up and wanted to share my experience and examples of improvements and changes for others who are coming from a "I want 2.8mm cameras everywhere to see everything" system mentality. My ongoing "build/looking for advice" thread can be found here...
ipcamtalk.com