It is based on the amount of SP you chose to power down. Your SP holdings could keep increasing though despite the power down - although the 1/104th payment will be the same.

You can initiate a new power down at any time (at which point the 1/104th slice is recalculated based on the SP amount you hold at the time when you click the button). So in this way you can increase the SP amount withdrawn (if in the meantime your SP is growing faster than your withdrawal rate).