I still don't understand why you'd want such an inaccurate health display, but any addon could easily be modified to do this. Figure out where it sets the health bar value; it should look something like this:
Lua Code:
local currentHealth = UnitHealth(unit)
local maximumHealth = UnitHealthMax(unit)
healthBar:SetMinMaxValues(0, maximumHealth)
healthBar:SetValue(currentHealth)
Change it to something like this:
Lua Code:
local currentHealth = UnitHealth(unit)
local maximumHealth = UnitHealthMax(unit)
local percentHealth = currentHealth / maximumHealth
local nearestTenth = ceil(percentHealth * 10) / 10
healthBar:SetMinMaxValues(0, 1)
healthBar:SetValue(nearestTenth)