Jump pack usage is particularly relevant to Blood Angels players.
This can be found in the Unit section under Jump Units.
Angels of Death have the complete breakdown on this here.
To actually get into assault, it is always better to use your jump pack in the movement phase.
What if things are looking a little desperate and it is time to make a calculated gamble to get off Hammer of Wrath? Up to 5 inches is pretty much a lock. 6" is still over 90%. 7" drops it to over 80%. 8" will drop it to 65% and everything else is under 50% after that. Unless you think it will be last turn or a way to kill a unit to take the objective, those Hammer of Wrath attacks are not that particularly strong nor valuable compared to the increased chance of actually making your assault.