Holley/Demon Carb stumble

You can leave the regulator at 9psi for the test.
How far open the ball valve is will determine what the system pressure is.

For the test, I would adjust the ball valve so the gauge reads 6psi.

If you want, you can do the test for both 6 and 9 psi....... it’s just a matter of closing the ball valve a bit farther to maintain the 9psi system pressure.
Be interesting to see how much lower the flow is at 9 vs 6 psi.