You are assuming that all 25 people are DPSing which they won't be
EDIT:
ok, the calculation does not take into account how much the unbuffed raid dps drops when someone dies.
thus, the effect when someone who was doing any dps dies is even bigger :)
but like said, the point of calculation is to show, that by dying, anyone drops the raid dps A LOT
(raid dps drops ~7-9% as you see from calculation / each dead), as the stacked buff from the charges is the key of the battle, even if the one that died did 0 dps.
+ raid dps drops by the amount of dps the one who died was doing.
to continue the number crunching...
average dps means average dps of the raid, no matter how many actually do damage.
if healers (8) do 0 and tanks (3) do half of what dps do, the "actual" dps must do a bit over 2400 to achieve average dps of 1500 in 25 man raid.
average dps of 1500 has nothing to do what is needed on Thadd btw. it was just used to do the calculations as a nice and round number.
and the stacked buff is also the reason why + and - charged people must pile up together (we would be spreading out if it wasn't needed).