NKF, on 21 July 2018 - 06:39 AM, said:
After a little digging around the sequel.... it seems to be:
UFO escape time - (Difficulty * 30) + Random number between 0 and UFO escape time
Difficulty will be the same difficulty level value that is used for calculating stats. So a Small Scout should escape on beginner (diff: 0) between 200 - 400 and on superhuman (diff: 4) around 80 - 280.
That fits nigh-perfectly, though only when taking into account something I just learned from deliberately lagging the game to get a frame-by-frame look. Specifically, interception starts at 80km (640 range units), NOT 75km as the UFOpaedia says.
So to close to 52km (Plasma Beam range) takes 112 frames, giving the Small Scout a 15.9% chance to run before the first shot. To close to 30km (Stingray range) takes 200 frames, giving the Small Scout a 59.7% chance to run before the first shot (although since the Stingray takes a significant time to reach its target, it may run before the missile hits). And to close to 10km (Cannon range) takes 280 frames, giving a mere 0.5% chance that an Interceptor with Cannons will get to fire (it won't hit, though, since the Small Scout runs at exactly the same frame the Cannon fires).
Among other things I've discovered from exhaustively testing interception: a mutual kill is possible. That is, it's possible that the X-Com craft is destroyed and
the UFO crashes (leaving a Crash Site and no X-Com craft after the interception window closes). Found that out when pitting a Firestorm against a Battleship a few dozen times.