Here's a simple and effective method to determine Zone I/ film threshold - density.

1. Shoot 5-6 blank/unexposed frames.

2. Shoot a series of exposures around Zone I (= -4 stops below grey card base meter reading). Shoot from 1 stop below Zone I, to 2 stops above Zone I, using either 1/2 or 1/3 stop increments. Shoot the rest of the roll at Zone V, as this helps to simulate a normally exposed roll of film.

3. Develop at products' recommended "standard" time.

4. use the unexposed/clear strip of film to make a test contact exposure strip, to find out the minimum amount of time required to get a maximum paper black.

5. once you have established the minimum contact time required for #4, then contact print your Zone I exposure test (#2)

6. If your minimum contact time for #5 is correct, then the frame from your Zone I exposure/threshold test, which prints as the first perceptible grey above paper black, will indicate both adequate Zone I threshold exposure, and also your corrected E.I. film speed. For example: You shot APX 400, but the frame that printed as the first perceptible grey above paper black was exposed as Zone I + 1 stop. This means that you needed to double the exposure to acheive a correct Zone I density, 4 stops below Zone V. therefore your corrected film speed is E.I. 200.

Of course, that's only the first half of the testing process, as determining a correct development time is the other half of this matter.

Good Luck!