Planned Hours = Duration * User's scheduled hours per day * % allocated per assignee
Example: 3 Users with open schedules, 8 hours per day, assigned 100% to a Task with a 1 week Planned Duration, means the Task would have 120 Planned Hours (8 hours a day per person = 40 hours for the week, 3 users x 40 hours = 120 hours.)
Planned Hours = Duration * User's scheduled hours per day
Example: 1 or more Users with open schedules, 8 hours per day, assigned to a Task means the Task would have 40 Planned Hours (8 hours per day, 1 week = 40 hours.)
% allocated towards Task from each resource doesn't appear to matter at all.
The actual formula is a bit more complex, being Planned Duration = (Planned Hours / (each assignee's % allocation)) / 8
However, it's easier if we go off my example above.
Workfront auto-populates Allocation % per assignee from Planned Duration / Planned Hours
Example: 3 users assigned to a task, 1 week Duration, 40 Planned Hours, each User would be allocated at 33.333.. %.. because they each have 40 hours available per week, only 33.3% of the time will they need to work on the Task.
Planned Hours are not related to Duration which is equal to what the User enters in the Duration field.
Planned Hours are divided evenly amongst all assignees
Example: 3 Users assigned to a task, 1 week Duration, 10 Planned Hours, each assignee gets 3.33 hours.