As you assumed, new shields mitigations are :
- common shield : 10
- rare shield : 20
- very rare shield : 30
You can see those values in the app when upgrading shields.
Note that those values are for newly obtained shields, old one will still have old mitigation's values.
The mitigation of the shields is a percentage of absorption of damages. A (common) shield with a mitigation of 10 will absorb 10% of the damages made by the buster. A second (common) shield with mitigation 10 will catch 10% of the remaining damages.
For example, let's imagine a burster making 100 of damages on a resonator. First common shield will absorb 10% so damages will be reduced to 90. A second common shield will absorb 10% of the 90 damages and will reduce damages to 81...
In the worst/better case with 4 very rare shields mitigation 30, a burster making 100 damages will be reduced to 24.
To compute the total mitigation of shields on a portal you have to just multiply the percentages. A very rare shield will eat 30%, to compute the effects you'll have to do
X - (X * 30 / 100) that will reduce to
X * 0.7.
So a very rare shield is equal to a 0.7 factor applied on the burster's damage, a rare shield is equal to 0.8 and a common to 0.9.
So the way of computing damages is :
RESULTING_DAMAGES = BURSTER_DAMAGES * SHIELD_1_FACTOR * SHIELD_2_FACTOR * SHIELD_3_FACTOR * SHIELD_4_FACTOR
And no matter the order of the shields, the result of a * is always the same :)