I struggled with the XP rate in my map for a while, and finally decided that rewarding XP via triggers and setting XP gained percentage to 0% worked best.
So:
Unit dies, pick every unit nearby, give XP based upon your function (so in your case, use some conditions to make determine if its level 10, 20, etc.)
Doing XP this way allows for it to be more easily controlled, imo.
I based the base XP reward upon the units level, then multiplied it by some value. This would require you to go back and edit unit levels, but then you can specifically determine its XP value, which is nice.