Dear anindya,

You can consider my understanding of this reaction force derivation in
http://www.coade.com/ubbthreads/ubbthreads.php?ubb=showflat&Main=4654&Number=21003#Post21003

The reactive force has two components, derived from Fluid mechanics model.
Reactive_Force= [mass flow-rate]*[jet_velocity]+ [p_jet]*[area_jet]

The first term has that expression which appears as an "enigma" for a regular engineer. However, jet_velocity is just the critical speed when the jet gas flow has Mach=1; the expression is not exactly the sound speed in resting fluid but is closed to that value- anyway this is just a detail.
The second term, even familiar, cannot be evaluated without a fluid mechanics model. The "best" is to have a result from a flow simulation, but depending on the application you can see some theoretical results in some articles.

So in my understanding both terms are to be considered.

Sorry for the answer delay!
Best regards.