I don't understand your test procedure. You put the function I gave you in your code, run your program and then do your test. Am I missing something?
An uninitialized CC2540 is going to have a 0db output. That is the default setting.
If you are not already running SmartRFStudio, use it to set the power level and it should remain set at that level.
If you are using HCI Tester, then write a simple script that will execute your next step after the the test command ends.
Write a test procedure and load it into the chip.
Thanks,