The range bug is Halo will completely miss at around 27.5 yards - 30 yards which should actually be around the sweet spot for max damage. I have submitted a Bug Report here if people could please bump it (Sorry I can't post links yet):
/wow/en/forum/topic/6712712994
For Halo range addon, try HaloPro found on curse.
There is also an active discussion on the addon at HowToPriest.com in the addon section