Russia says the SU 24 was 6,000m amsl, 1,000m lateral distance from the border.
Being so confident of your position as to get that close to the border in the Su 24 is an awfully big (crazy?) gamble that the F16s are equally confident of their position (and yours) in deciding shoot/no shoot.
Even manoeuvring at only 350kts, 1000m is c 5 secs difference between being dead-right and dead-wrong about which side of the border you're on.
But either way, its close enough to end up being dead.