While my method doesn't go to the lengths of exactness of the methods some folks use, it's simple and doesn't require expensive tools. I call it the Fred Picker modified by Michael Smith method.
First find the printing time to reach maximum black on Azo with your printing set up. To do that, make a test strip through an unexposed but developed and fixed negative. The time required to make the last noticable strip is your proper proof time.
Then expose negatives of a full scene at various speeds - 50, 100, 200, 400 - something like that where you go above and below the manufacturer's speed. Develop normally. Make contact prints of each at your proper proof time. You will be able to see the one that is correct, or be able to make a reasonable estimate of your personal film speed.
As you make additional photographs, adjust if you think you need to. Michael has written about this.
juan