www,
If I wait until the aircraft are 1nm apart before calling traffic, it's going to be too late if they haven't spotted each other already. Do the maths: Interpret my message; visually acquire the other guy; decide on avoiding action; wait for aircraft inertia to take up new heading .......... Bang!
Two aircraft on reciprocal tracks, same level, could easily be a definite risk of collision when they're 10 miles apart. At what range would you step in with traffic info?