Discussion
Theft, slavery, exploitation, coercion, and surly fraud. 
We live in society. Those that live in the middle of nowhere, without connection to society, that do not use anything from society, can't earn much really. Best they can hope to do is to be self sufficient and hope for best in extreme situations (drought, fire, flood etc.)
You don't see many millionaires (or equivalent of) that came out of the woods with a lot of wealth that they created while being disconnected from society. That's simply not how world works.
And when you use society, than you do own something to it. How much? It's hard thing to define. Best i can do is : "it depends on how much society you live in gives to you". So it's not one fixed amount or percentage. It depends on society you live in. If it offers a lot, you own them more than if it does not offers a lot. So same percentage (let's say 20% of what you make) can be way way way too much, or way way way too little.
For example, i am not glad we (my countryman) have to pay 38% of average paycheck to my country. I think it should be less. Not because i think that 38% is too big number, but because i think that my country does not offer enough to warrant those 38%. It's not far, but i think it's too much. On the other hand, there are certain countries where i would be glad to pay 50% or even bit more and there are certain countries i would not be happy to pay even 10% and there are some that would have to pay to me to live there.
So answer to question, "what is fair share" would be "depends".