5/5/10 works.
by the way, it's not 5/10/15 per tier for the item, it's 5 at levels 10/15, 10 at 20/25, and 15 at 30...
So it's more like 0/5/10, 15 at 30. Not as good.
But you're right, 5/5/10 is a very good way to go about it - it was my first idea, but I argued myself out of it :P