Voltage Drop Across MOSFET in Ground path
an NMOS is connected in the ground path the control the power flow


It's worth mentioning that when the FET is "off" current will flow through the diode - is this what you really want?

If so then just do as pjc50 says and treat the MOSFET as a resistor equal to its on-resistance.

Looking at your circuit a bit more, all it realistically does is protect the load from reverse polarity inputs.