Hi Chris. In such "part time worker" cases, I'd suggest setting up a 20 hour Per Week Schedule and attaching it to the user in question. When the user is then attached to the Task (e.g. as Primary Assignee), Workfront is smart enough to recognize that the User's Schedule has precedence, and "gear back" the rate of work accordingly. Conversely, if you then took the Task away from that resource and assigned it to a different resource (either one with the Default 40 hour per week Schedule, or WITHOUT a Schedule, in which case, the Project Schedule then takes precedence...or, lacking one, the default Schedule then kicks in), Workfront would recompute the duration at a 40 hour per week pace, as you're seeking.
Regards,
Doug