It's pretty simple. The hardest part is converting the old speeds to current values. If you were to use a 100 speed film a direct value would be 100 @ 16.


Make your ISO a fraction and read it like this.......1/160th your f stop is still 16 for bright sun.
ISO 25 would be 1/25th @ 16.
This works for any speed film. You just need to use a speed that's close. On your camera that's 1/100 @ f22 or 1/300 @ f11.
That's ball park or calibrated eyeball(sunny 16)

Redundancy not intentional.