Zackly. 400' is very simple to determine. If you have a private pilot at your field he can tell you how high it is. It's 1/2 distance of pattern traffic.

Also you only need to be below 400' within 3 miles of an airport, for obvious reasons. If your field is not within 3 miles of an airport, fly as high as you want so long as not to interfere full scale planes.