Hello,
the datasheet only gives the values for 0mA: 6.0V and for 10mA: 7.5V.
The series pass transistor is usually a MOSFET with some on-resistance which will be more or less constant.
The 10mA range equals to 1.5V overhead voltage. In my opinion the rule of three is a way to approximately calculate the necessary input voltage, which would be 6.6V and with some safety: 6.8V.
Regards,
Reinhard