Statistical counting sensitivity equation

[dmlibguard]

Hello,
I have been tasked with determining the statistical counting sensitivity for our sealed source wipe tests. The equation i have is from an old FEMA Civil Defense manual and we have some concerns about the procedure. Is there another reference that explains how to determine the sensitivity? Any Help is appreciated.

Our counting setup is a Ludlum 2200 scaler with an end window gm tube(same tube as a Ludlum 44-7) in a well.