most people don't really know what the trap tension is set at. Its almost impossible to accurately tell because tension is only one variable. On most coyote sized traps, a pan that just stays up by itself, when set will have about 2 1/2 to 3 lbs of middle to free end tension.
I've tested many traps with a tension tester, and this is pretty close more often than not.
and thats about right for all canines, including fox. When you add dirt, you decrease the "tension"
at roughly 3# tension, I catch every fox that walks on the pan.....trappnman