The formula is linear, so a missile with a 100% chance at 10000km/s will have a 25% chance against one going 40,000 km/s. Something that really helps when you get to higher tech levels is to look at the agility rating of the missile (what shows on the missile design screen, not how many points of space you used). This is actually the chance to hit a target moving the same speed as the missile moves. So a 30,000km/s missile with an agility of 40 would have a 40% chance against a missile moving 30,000km/s and a 20% chance against one moving at 60,000km/s.
Hope this helps you
Brian
