Is the "one finger rule" i.e. finger able to fit inbetween the scrotum and the ring a good guide?

I would say it's a good starting point. You may need to go slightly larger if your balls start turning a darker color, or if urine flow is restricted. Slightly tighter if your cage sags.

 

Which finger is it, if its your little finger you should be about right, though clearly everyone's anatomy is different and it depends how relaxed you are when sizing.
I agree a with previous comments as long as you are not going a dark shade of purple when attempting an errection you should be OK. Also your body adapts over time so what might seem tight when you start might seem looser later. I have reduced ring size by 4 mm over several months.

 

if you am new and only just have start to wear your cage then it migt hurt a bit till you get use to it. if you can get a little finger in it its ok.

 

I have gone from CB 600 to birdcage, to Htv2. Now thru Ali Express there name A271 short 45mm cage. From my point of view I have noticed that longe in a cage the smaller the ring I can wear.

 

That makes sense. I've noticed the same thing. If you sneeze a balloon on one end the other end expands to compensate. In my experience the same holds true with a dick cage and an erection. With a smaller tight fitting cage, the swelling behind the cage close to the body will expand. As long as you can urinate freely without obstruction a smaller cage is usually more comfortable.