I far as i am aware there is no % attached , each drop list is random on mobs death items are picked from the drop file at time of death.
to test this take a mob say a scarecrow spawn 100 of them making sure that no other scarecrows spawn , set the drop file to something you can recognize e.g.
1/100 sunpot
1/20 halfmoon
1/2 medhealthpot
Then kill hundred of them , then count how many of each items drop , you be surpised ---- I did this test had 13 sunpots . so my feelings are that the dropfile is the main interger in what it drops not % of mobs killed . which would make sense because you would have people ingame counting deaths of say boss mobs and working out which boss to kill at which time ie 100th time to get a certain item....
It was an old misconseption i think in the euro mir community , bit like card counting in casinos...
If you want a specific drop list say out 100 mobs you can spawn 99 of 1 type of mob with 1 drop list and set it to spawn bonus mob at 100 but with a different drop list i guess.
I maybe wrong ...