It’s very simple, point the arrow at your iso and then trigger flash. Where the arrow points you cross reference the chart.
I might ring the number one day!
Probably only worth doing if you are interested in double glazing or felt-based artworks!:
https://www.pinnaclewindowsltd.co.uk/contact/
http://www.feltable.com/contact-me.html
The area code dates it to before they added a '1' in 1995, and the meter looks older than that. There's a newly built house at the maker's address today.
From the finish of the dial labels, I'd guess a small operation, perhaps just an inventive bloke with a workshop. Great that it's still doing the job!