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.