So, I guess I don't understand SDI, what is the formula? I can't seem to find it. I would have thought it would be simple. If you have 50% SDI only 50% of missiles sent to your country are successful, however this is never the case.

50% chance to deflect missile not 50% of missiles deflected, I got once missiled few times and had over 40% sdi and all missiles got thru.

exactly, that's not 40% to me

It's probability.

Just a normal distribution of the missiles getting through, with the root mean square being 50%.

even if you have 90% sdi and you get missiled 10 times its possible that all/most get thru.

It's kind of like flipping a coin ... if you flip 9 times and 6 times heads came up and three times tails came up what are the odds for heads coming up on the next flip? .... it's still a 50/50 chance of heads coming up.

Maybe you have just been unlucky. Also, with SDI you don't even get the same chance each time because every missile lowers SDI a bit.

I thought only missiles stopped lowered the SDI.

Every wonder why you stop missiles when you don't have SDI tech and it is still at 1.00%? Same thing, probability.

or you don't get op on 0-1 spal country when you have 50+ spal.

Does the Luck Bonus affect this?

maybe but finding it out is basically impossible due randomness (15% sdi can stop 5/10 missiles or 10/10 get thru).