I think you would need to pick up a textebook on gas turbine design. I suspect the blade open area was designed for optimium efficiency, and that occurs when the relative velocity between the incoming gases and the blades is equal to 50% soundspeed. If you know the gas's inlet T and molecular weight, you can calculate the soundspeed then calculate the required open area and bladed speed for optimum efficiency.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

I think you could look at the basics of how a flow element works. There is a relationship between variation of pressures between pressure taps, the flow element geometry and mass flow (or volume flow).

You need to check this reference and look at the nozzle type of geometry for flow element:
Flow Measurement Engineering Handbook, by Richard W. Miller.

Otherwise, my guess is that if there is an obstruction downstream, it would create a back pressure which can be detected by reduction of flow thru the nozzle from a departing value. But if your pressure upstream increases at the same time, say because you can control an equipment that does this, then the flow would go up again even in presence of the obstruction. Would that make sense?

Sorry I just had a second thought while reading the subject...
Are you referring to the main inlet of turbine or just doing a flow measurement on a certain pipe equipped with a nozzle?

A read of page 10-13 with equation 10-20 for flow through venturi nozzles in Perry Chem Engg Handbook 7th edition may help.