Well, the sectioning will only ever work as an approximation.
Here is the reason why (several examples)
1 damage vs. 2 damage Equal utility for a 1 HP creature, unequal utility for a 2 HP creature
Healing 10 HP vs. Healing 20HP Equal utility at 90HP, unequal utility at 80HP.
Need I go on?
Trust me that the uniqueness of these abilities means that the grouping will help us only to the point of being a useful approximation. We are not going to produce a formula that has mathematical reasoning behind it.
We can figure out one that would be a useful guide, but it will still have to be adjusted for a given scenario.
EDIT: Not necessarily over .5 quanta. Like I said, this will ultimately have to be a case by case thing. However, I was arguing the mathematical principle behind it, which says there should be some sort of decrease. Whether or not that gets nullified by rounding was not part of my original consideration