For a graphical interpretation of this equation, see here.
This generates roughly the following:
10% damage/healing at 0 yards
30% damage/healing at 13 yards
74% damage/healing at 20 yards
90% damage/healing at 22 yards
100% damage/healing at 27 yards
89% damage/healing at 30 yards
The rough effect of Halo's scaling is that its effect is weak on targets near to the priest, but has a 'sweet spot' at a radius of roughly 20-30 yards, where it will generate substantial damage/healing. Priests should ideally therefore try to place the desired targets at that range before using Halo.
Tips and tactics
It can be very useful for instantly dispatching large numbers of low-health foes.
Patch 7.0.3 (2016-07-19): No longer available to Shadow priests. Now deals 32% more damage.
Patch 6.1.0 (2015-02-24): Now displays an indicator on the ground to make it easier for the casting Priest see where the ability is most effective. Additionally, Halo's visual effect has been synchronized so a character will receive the effect when the nova intersects with them.