Perhaps something like...
If spy mission succesful then:
10% chance it goes to current DEFCON level +1
20% is goes to current DEFCON level +2
30% chance it goes to current DEFCON level +3
40% chance it goes to curent DEFCON level +4
If the new DEFCON value exceeds 5, then it just goes to DEFCON 5
So;
- If your enemy is in DEFCON 4, if you suceeded, there would be a 100% chance it goes to DEFCON 5
- If your enemy is in DEFCON 3 and you succeeded, there would be a 10% chance it goes to DEFCON 4, and 90% it goes to 5
- If your enemy is in DEFCON 2 and you succeeded, there would be a 10% chance it goes to DEFCON 3, 20% chance it goes to DEFCON 4 and and 70% it goes to 5
- If your enemy is in DEFCON 1 odds as per above.