Voltage drop over MOSFET
Welcome to the miraculous world of low Rdson MOSFETs.
Yes, the results you have calculated will be  ABOUT correct.
Data sheets give typical values in graphs and large print claims unless otherwise noted.  At high power levels where the device is heated significantly by the switched current, actual Rdson can be up to about twice the 25C value for SOME devices. But at low currents compared to Idsmax, the Rdson will not vary much during operation due to heating.

Look at the graph at top left on page 3 of the data sheet. This shows the device current / voltage drop characteristics for various gate voltages. The Vgs = -10V "curve" is close to a straight line and intersects Vds = 1V at about Ids = 16A, or Rdson = 1V/16A =  = 62.5 milliOhm =~ 65 mOhm as claimed.

Your chosen MOSFET is able to easily carry 3A or about 25 x your design current - so is not at all stressed and is liable to run at near ambient temperature. 
Thermal resistance on FR$ PCB is 166 C/W worst case.
Device dissipation at 120 mA and say 80 milliOhm Rdson is
P = I^2R
= 0.120^2 x 0.08 =  1.12 mW
for a temperature rise of aboyt 0.0012 x 166 C/W =~ 0.2 C :-)