In XCOM: Enemy Unknown, this post mentions how defensive bonuses are calculated with regard to the amount of cover, perks and additional buffs.
For XCOM 2, I currently have this defensive bonus list:
- Half Cover: +20
- Smoke: +20
- Hunker Down: +30
- Half Cover w/ Smoke: +40
- Half Cover w/ Hunker Down: +50
- Half Cover w/ Smoke and Hunker Down: +70
- Full Cover w/ Smoke: +60
- Full Cover w/ Hunker Down: +70
- Full Cover w/ Smoke and Hunker Down: +90
What perks or armors are available to further add to these bonuses? For example, in XCOM: Enemy Unknown, the 'Dense Smoke' perk allowed smoke grenades to give an additional +20 defense.
Best Answer
After looking at a number of abilities and @Sumurai8's comment, I've compiled that the following list of defense bonuses:
Maximum defense can then be gained by using Full Cover, Smoke, Hunker Down and Gremlin MKIII Aid Protocol for a total of 130 defense.
Since defense is a number subtracted from the shot chance, at 130 defense it is not possible to be hit by a shot. Psi attacks and grenades may still hit.