Editing MathHammer (section)

==Re-rolls==
An ability to re-roll will multiply the number of results that result in a hit or a wound by (6+x)/6, where x is the number of facings on a hit roll you can re-roll, so if you re-roll 1s x is 1, re-roll 1s, 2s, 3s and 4s x is 4.

Chaos Space Marines within 6" of a Chaos Lord can re-roll 1s to hit so you multiply their number of results that result in a hit by 4*(6+1)/6=4.667

The efficacy of re-rolls cannot be understated, a Chaos Lord increases the damage output of nearby units by 17%, that means that if you have 6 times the value of the Chaos Lord in nearby units then the Chaos Lord will be effective for his pts simply based on his Aura, note that as you lose units the Aura will become less and less effective and Characters are often less durable for their pts than other units, so while a Chaos Lord is amazing while you have 10 times the number of pts within range of his aura, if you only have 2 times the number of pts within range of his aura he needs to do more work himself to justify his pts cost and inclusion in your army.