The ISO # is an international standard that should make cameras, films, sensors, shutters and apertures give you the correct exposure.
We know that your exposure determines the shadow areas of your negative, development determines the contrast.

Exposures can be divided up into steps of tone from black to white or clear film to dense film. These steps (zones) are broken up into divisions of one "stop" of exposure. This is often refered to as changes in film densities.

Meters (and ISO) are designed to give a middle gray exposure. They might not be accurate. This test is a check to see if this is true for your meter and film. If necessary we can correct this by simply setting our meter to a "custom" iso.

Film speed is determined by the exposure that reaches the first hint of shadow detail (Zone 1 density on the film.)
Clear film should give "maximum black" Shadow detail is the first hint of density 

This test is simply metering a test targetand bracketing exposures above and below the metered settings, set to give us zone one.

The target you photograph should be smooth toned, evenly lit, neutral in color and large enough to fill the frame.
Set your meter to the manufacturers film speed (iso#).
Point your camera at the subject and set the lens and shutter to give the recommended exposure.
Choose a shutter speed that requires an f/stop of approximately f/4or f/5.6. Don’t change the shutter speed.

1- Make your first exposure- this will be middle gray Zone 5. f/stop____ Shutter 1/_______

2- Make your second exposure Close your lens 4 stops- (count 4 full stops - eg. 5.6>8>11>16)
this should be Zone 1 f/stop____ S 1/_______

3) Make 4 more exposures:
Bracket the second exposure (Zone 1) in half stop increments - record the exposures
(eg. If 2nd exposure was f11, shoot f8, f8/11, 11/16 and f16)
+ ½ f/_______ half stop more exposure
+ 1 f/_______ full stop more exposure
-½ f/_______ half stop less exposure
- 1 f/________ full stop less exposure

Develop the roll normally- be careful to adhere to time and temperature with consistent agitation.

When the film is dry, evaluate it for the frame is the true Zone 1 exposure (first hint of gray on the film)tone above clear film.This frame indicates your calibrated film speed for your camera, lens and meter.
If the second exposure (four stops under the meter for zone one) is correct, you use the film speed recommended by the manufacturer.
- ½f stop is correct, increases the iso# (400 speed film goes to 600)
- 1f stop doubles the iso (400 speed film goes to 800)
+ ½ f stop reduces the iso (400 speed film goes to 300)
+1 f stop reduces the iso by half (400 speed film goes to 200)