I don't know that I would say there's an "ideal" weight to go to the billy. It depends on the size of the goat- a big goat will weigh more and might be 70kgs and be fine, and so might a 45-50kg goat- just that it isn't as tall.
I don't normally weigh all my goats- it just so happened we were at a show at Forfar market in August, and my neices and some children found if they sat up on top of the hurdles they could see into the weigh bridge hut and read what weight the scales said. So they weighed most of our goats in it. It is interesting having a definitive weight as opposed to just our guesses. I wouldn't probably have save Raker is as heavy as he is, that was a bit scary!
Sitting down chewing the cud is indeed a sign of a happy healthy goat.
Beth