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.