Well first of all that rate 300 is pretty high. That's 300% more EXP each kill. That is quadruple EXP the first kill! Very abusable. I'd rather put something like 10-30.
Second, monsters level up to "max base level" of the PC they killed, so it depends what the max level is on your server.
You can change it by putting concrete number in pc.c:
if(battle_config.mobs_level_up && md->status.hp &&
(unsigned int)md->level < pc_maxbaselv(sd) &&
!md->guardian_data && !md->special_state.ai// Guardians/summons should not level. [Skotlex]
) { // monster level up [Valaris]
clif_misceffect(&md->bl,0);
md->level++;
status_calc_mob(md, SCO_NONE);
status_percent_heal(src,10,0);
if( battle_config.show_mob_info&4 )
{// update name with new level
clif_charnameack(0, &md->bl);
}
}
Just replace "pc_maxbaselv(sd)" with whatever max level you want.