I have a broken screen, with only the upper left corner usable. I use Openbox as my window manager (through Crunchbang). I have this at the start of .config/openbox/autostart (but I suppose you can add it to anything that executes after X starts):
xrandr --newmode "1016x670_59.90" 54.87 1016 1064 1168 1320 670 671 674 694 -HSync +Vsync
xrandr --addmode LVDS1 1016x670_59.90
xrandr --output LVDS1 --fb 1016x670 --panning 1016x670 --mode 1016x670_59.90
xrandr --fb 1016x670 --output LVDS1 --mode 1280x800
This reduces my screen to using a 1016x670 box in the upper left corner. I have no idea why I have to create the new mode and switch out of it again, but this works for me.
I got the modeline from gtf:
gtf 1016 670 59.9
In your case I think you're aiming for 1160x568 (200px padding all around).
Get the modeline:
gtf 1160 568 59.9
>> # 1160x568 @ 59.90 Hz (GTF) hsync: 35.22 kHz; pclk: 52.13 MHz
>> Modeline "1160x568_59.90" 52.13 1160 1200 1320 1480 568 569 572 588 -HSync +Vsync
Then
xrandr --newmode "1160x568_59.90" 52.13 1160 1200 1320 1480 568 569 572 588 -HSync +Vsync
xrandr --addmode VGA 1160x568_59.90
xrandr --output VGA --fb 1160x568 --panning 1160x568 --mode 1160x568_59.90
xrandr --fb 1160x568 --output VGA --mode 1280x800