Hello to all

I know somebody here will know this-----
What EXACTLY does 12/1 mean on barrel flat of english guns??
WHAT does 13/1 mean
ditto 15/1

I realise it relates to the bore, 9" from the breech , but what are the sizes
How do I dertimine mathematically the 13/1 and the 15/1 sizes from the standard .729 of 12ga??
I THINK????? it relates to the number of those sized ball of lead to the pound?
Is this correct

Cheers