You're very close. If you set the task as Fixed Duration the next step is to display a view that allows you to enter the work and let Project worry about the assignment units.



From the Gantt chart view, right click in the graphic portion (right side) and select "Show Split" from the shortcut menu. This should show the Task Form in the lower portion of your screen. Select the task, click in the task form under "Resource Name" and select the resource you wish to assign. Click in the Work column and enter the 2 hours of work. Click OK.

See:
http://blogs.msdn.com/b/project/arch...ject-2010.aspx

for a discussion about the assignment units not showing correctly in the Task Form. Although Heather's post talks about 2010 - the same applies to 2013.

Using Fixed Duration task and assigning work is much easier than you doing the math to calculate assignment units.