ISO Speed = 0.8 / (Exposure st speed point)
The speed index result is rounded off to the nearest 1/3rd f-stop.
For color reversal films, the the exposure needed to get to the shoulder point (S), which is about 90% maximum density is determined. Then the exposure needed to get to the toe (T) which is 0.2 density above the minumum density is measured. These two exposures are averaged, that is: ((S+T)/2), and this value is the Speed Point. The reciprocal of the exposure at the Speed Point is then multiplied by 10 and the result is the arithmetic Speed Number.
The preceeding was taken from "Sensitometry for Photographers" by Jack Eggleston. Stephen recommended this book to me many years ago, and it is certainly worth getting if you are at all interested in the subjects at hand.