How To Do Right Drop Rate

Like my topic title said " How To Do Right Drop Rate "

1) how can i do an random drop ? i mean i have to edit CommDrop or Mobs Drop?
2) i used CommDrop and i used 2 items with the same id but option re different' and i get bug(bug means only alz drop) i guess that i have to use MobsDrop if i want a random drop,right guys?
well...here we go MobsDrop
[MobsDrop] TerrainIdx SpeciesIdx ItemKind ItemOpt DropRate MinLv MaxDropCnt OptPoolIdx DurationIdx

can any1 learn me how to do that.. i mean can any1 tell me what Terraindx,Speciesidx,etc should i use..and abut that DropRate ...how can i generate that code?isn't with 20%/30%,etc%..right?
@ALREADY USING "UseWorldDrop=1" IN WORLDSVR_XX_XX.
please help my i really need it,appreciate this!

thank you guys