How about you have a base evasion/intercept chance... any new promotions is factored as such:
promotion value % of (100% - base value)
so if you have a 60% evade chance and a promotion that gives a 50% bonus to evasion, rather than equaling an 110% evasion chance, it would be 50% of the remaining 40% difference (which is 20)... so in this case, it would be: 60 + 20 = 80% evasion chance.
Then if you add an additional promotion, the value is the percent of the remaining difference between the current evasion chance and perfect.
I've seen this system used in other games, and what's good about is that you can never actually reach 100%... so even if you want to add in things later like RADAR towers that would give an extra promotion or airbase improvements, you'll never reach 100%. But at the same time, you won't have to really lower the values of the promotions either...
Because it would stink if you had all of these different promotions, and consequently, your first promotion takes you from 50% evasion chance to 55% or something. So you'll get a big boost from any one promotion and gradually smaller bonuses from each additional promotion on top of that one.