Modified from hyperbear.com:
The armor rating is calculated after any armour dice shifts. A figure in D8 armor behind hard cover would have D12 armor, which results in an armor rating of 6.
A D12 Power Armor figure behind soft cover already has the maximum die possible for armor, so it would continue to have an armor rating of 6.
One change must be made to the artillery rules. Give anti-armor submunitions a die shift UP 1 against Power Armor. Instead of having a D6 impact against Power Armor (as they are considered a dispersed target), they now have an impact of D8 against Power Armor.
The combat resolution system remains the same up to, and including, the calculation of the number of potential casualties. The attacking player rolls a number of impact dice equal to the number of potential casualties. These dice are compared to the armor rating of the figures in the squad. If an impact die exceeds the armor rating, one of the potential casualties is wounded. Otherwise, the armor protected the potential casualty.
Note that the status of wounded figures — killed, injured, or okay — is unknown until the squad conducts a Reorganize action in order to treat the wounded. Exception: as usual, Power Armor troops test for the status of wounded figures as soon as the PA trooper is wounded.
In addition to this, any IMPACT DIE result that scores TWICE OR OVER the ARMOR RATING is a kill.
Thus, a shot from a basic assault rifle (D8) which strikes a soldier wearing Combat Armor (AR 4) has a 50% chance (1-4) of having no effect; a 37.5% chance (5-7) of wounding and a 12.5% chance (8) of killing the target.