I don't get many things right the first time, in fact, I am told that a lot. - Ben Folds
So looks like I took another hiatus from 40K, luckily (a word I never use in conjunction with mathhammer) I'm back and brought an easy way to factor in re-rolls to Method 216.
Method 216 is a super easy and simple method for calculating expected kills in Warhammer 40K using some simple math in your head, on some paper, or even using the calculator on your cell phone.
If you don't know what Method 216 is, or just don't remember, then go learn all about it here Super Simple Method 216.
Now that you are a Method 216 expert, let's push it a step further and factor in the re-rolls.
Here's an example to start us off,
8 Guardian Jetbikes (with no Shuriken Cannon upgrades) are shooting at a Space Marine tactical squad.
The calculation for Method 216 would be,
(16 * 3 * 3 * 2) / 216 = 1.33~ Space Marines shot to pieces!
BUT WAIT A MINUTE, those Shuriken Cannons are twin linked, so how do I factor in the re-rolls?
For re-rolling failed hits and wounds all you have to do is add the Method 216 result from before times the re-roll multiplier in the chart below.
So since the Guardian Jetbikes needed a 4+ to hit before, you take the 1.33~ dead marines and add 1.33~ times 0.50.
1.33~ + (1.33 * 0.50) = 1.96~
An easier way to do with is simply 1.33~ * 1.50 = 1.96~ Dead Space Marines
If a Farseer had Doomed the Space Marine Tactical Squad, then the calculation would be,
1.33~ * 1.5 * 1.5 = 3~ Dead Space Marines
Now then for re-rolling saves you simply multiply the Method 216 result by the re-roll multiplier.
So if the 7 remaining marines were to rapid fire back (assuming they all just had boltguns) then you would have,
(14 * 4 * 3 * 2) / 216 = 1.56~ Eldar souls eaten by the warp
If the Farseer had cast Fortune on the Guardian Jetbikes then it would look like.
1.56~ * 0.33 = 0.51 Eldar souls eaten by the warp
Now go forth and mathhammer!!!