What percentage of my animal will I get back?

You’ll get all of it back? However unless you want to be really creative and make  bone broth, soup, dripping or lard or soap from the waste you will have quite a significant amount to dispose of. Here is a useful link explaining the amount of waste from various carcasses. 
https://onpasture.com/wp-content/uploads/2014/11/The-Butcher-Stole-My-Meat.pdf

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