The window opening % in the DEF setting, will reset to 0% in each of these cases:
1. each time BERS is restarted
2. each time a level is added
The reason behind this is that Default settings for windows are by design to apply the most common attributes to each zone with the assumption that styles and dimensions and opening % are likely to be changed. The opening % is therefore reset as described because it is not a permanent setting.
The workaround is to set the opening % in each of the zones as Default window settings are applied.