Will answer to myself. Probably this may help:
Brightness for plugged power:
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Power\PowerSettings\f1fbfde2-a960-4165-9f88-50667911ce96]
"ACSettingIndex"=dword:00000005
Brightness for battery:
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Power\PowerSettings\f1fbfde2-a960-4165-9f88-50667911ce96]
"DCSettingIndex"=dword:00000005Both values in percents. dword:00000005 = 5%