I have searched but have found no settings in Windows 8 for battery charging - the battery is designed to charge up to 100%.
Some manufacturers (including Sony and Lenovo) provide a utility which limits the battery from fully charging, but I have not been able to find any such utility by Microsoft for the Surface Pro
or for Windows 8 in general. The advanced Power Options seem to be the only available mechanism.
A look at the WMI Win32_Battery class has found that one can consult the battery state
or set the desired power state as in Power Options, but not much else.
A visit to the Apps for Windows website has found nothing as regarding battery charging.
It seems to me that unless Microsoft will improve
battery-handling in Power Options at some future release,
if you wish to stop charging the battery before 100% then your only choice at the moment
is to unplug the adapter from the mains.