Exact values for heal + group heal aggro?

[TheFishyOne]

Anyone have the equations for calculating how much aggro a successful heal generates? How does overhealing factor into this? Also, how does the aggro modifier for group heals work, from a technical perspective?

I sometimes use group heals in 6 man dungeon crawls, and would like to have an idea of how much aggro the tank should have to safely use one vs popping DA immediately afterward. I'm also looking for numbers to determine if Word of Resto is safe to use on Trak to cleanse my group after his DoT.

Thanks!