There are two ways to reduce - possibly even eliminate - overscan/underscan. First, get a rock-solid eye pleasing display; don't bother trying to eliminate overscan/underscan until you're satisfied with the display to begin with. Once you have a satisfactory, albeit overscanned/underscanned, display, try this:
1. PowerStrip menu > Display profiles > Configure > Advanced timing options
2. Check "Lock scan frequencies"
3. Use the arrow buttons to reduce/expand screen size and center
If that doesn't work - and it probably won't if you are tuning a HDTV which is designed for overscanned signals - or is less than satisfactory, consider using a so-called "resolution-in-a-resolution":
1. PowerStrip menu > Display profiles > Configure
2. Switch to the overscanned/underscanned but otherwise satisfactory resolution if you're not there already
3. Click Advanced timing options > Custom resolutions
4. Check "Lock total geometry"
5. Reduce/increase the number of "Active pixels" by the estimated amount of overscan/underscan, e.g., if you are at 1920x540 and there is ~ 5% overscan, change the number of active pixels to 1824x514
6. Click "Add resolution" and follow the instructions on screen
This second method, of course, only works if your display driver supports user-defined resolutions.