1 Volt = 2.2dBu
10 Volts = 22 dBu
100 Volts = 42
1000 Volts = 62
10,000 Volts = 82
100,000 Volts 102
1,000,000 Volts 122

5 Volts = 16 dBu
50 Volts = 36
500 Volts = 56
5000 Volts = 76
50,000 Volts = 96
500,000 Volts = 116
5,000,000 Volts = 136

half of 50,000 Volts is 25,000 so that would mean 90dBu. A decrease of -6dBu for half the power.