Easy as pie
Each drop has a dkp price, so for example some Z'G drops, [Seal of the Gurubashi Berserker] 16.5, [Runed Bloodstained Hauberk] 56.95, [Mar'li's Touch] 23.8.
So everytime someone gets them they loss the dkp stated, and the rest get the proportional part of that dkp, being a 20 man raid, and the one getting the item not gaining dkp but losing, each player gets 1/19 of the dkp price of the item. If nobody wants the item, it goes to bank, and then instead of 1/19, each player gets 1/20.
The idea is at the end the dkp gained/lost in the guild has to be equal to 0
You can have negative dkp, but obviously, if you ask for an item having negative dkp, will be easy for the rest of raid to overpass you
Example:
Player A has 10 dkp
Player B has -47 dkp
Player C has 53.5 dkp
Player D has 0 dkp
Player E has -16.5 Dkp
chechsum 0
So player a gets [Runed Bloodstained Hauberk] 56.95
Being just 5 players they would earn 1/4 instead 1/19
Player A has -46.95 dkp (-56.95)
Player B has -32.77 dkp (+14.23)
Player C has 67.73 dkp (+14.23)
Player D has 14.23 dkp (+14.23)
Player E has -2.27 Dkp (+14.23)
chechsum 0
For sure I've done the mats bad