After rolling and rerolls, change successes to critical successes. Fighters with the Enraged ability from Raging Slayers should set this to 1.

After rolling and rerolls, change misses to successes. The effects of cards like Out of Nowhere or Henchman can be modeled with this.

Dice Advice
for
Warhammer Underworlds

Calculate the probabilities of combat outcomes in Warhammer Underworlds 2nd edition. 

The success target is a conversion of the Underworlds combat dice to a traditional d6; lowering the number improves the accuracy of the attack. For example, lower the number by one for flanking and by two for surrounding.

Results are calculated with polynomial distributions. You can instead use a Monte Carlo simulation to calculate the results; they should be the same, but it's easier for me to implement the Monte Carlo simulation correctly.

Attacker

Defender