how do i tell if iam 400 foot
Stand back from your model a fixed distance say 20 feet, hold a stick in front of you a fixed distance say 2 feet. put the stick at the nose or tail of the plane and mark on the stick where the other end of the plane falls on that. Now if the plane was 10 feet in front of the stick then it will appear 1/40 the the size at 400 feet. So if the mark on the stick was 12 inches then the plane will be 20/400x12=.6 inches.
I think I have those ratios right, but if not jump in.