How to tell the correct weight for a cat?

Is there a way to determine the correct weight for a cat?

This is a large long-haired cat, taken over as a stray.
The hair is so long that you can't get near enough to its body to see if
you can feel its ribs.

With humans you can use BMI - but is there a cat equivalent?

It weighs 6.1 kg - but has enormous paws - which makes me think it
probably should be big.

Is there a reliable way of knowing for certain?
--
Chris