you can try to do it with npc script. OnMobKill:
if player base level >= 900
use some kind of formula that will give the player more exp in addition to the exp they gain from killing monster that will result in 1% minimum exp.
Although im not really sure if there's other way to do this.
you can change the exp given by the monster on 'mob_db.txt' found at the 'db' folder.
and if i'm not mistaken, you can only change the formula for aspd in the src codes not in some config files.
I don't know if there's any command to check if the tile is already occupied by another item. I guess there isn't.
If you wanted to randomly drop it just use rand.
sample
prontera,150,150,4 script randromdrop 100,{
makeitem 608,1,"prontera",rand(147,153),rand(147,153);
end;
}