Reverse-Current Protection with p-Channel MOSFET
Using a Pmos as reverse protection is good to avoid voltage drop across the switch.
To evaluate Vds we should start looking for rdson.
Looking at image 13 in the datasheet 
We can see that increasing Vgs will decrease the Rdson, and the resistance starts to increase when high current (5A) starts to flow in the pmos.
Increasing Vgs decrease the current dependence, considering your Vgs (-3.3V) means that you are beetwen these lines, therefore at your current you will have a maximum of 0.1 ohms (minimum of top line).
So a vds of 0.1mV