That's a very valid and hard question. You do immediately frame it in a non neutral way, which is unhelpful.
There are good reasons to still have a significant amount of personal freedom in spending earned money: we have an economic system that works faster with it. Citizens can vote with their wallets on matters where collective spending is not necessary, thereby steering development of technology (and society) in a certain direction in a crowdsourced way. Personal spending power is also used as a reward for doing things (although technically this does not have to be in terms of money in a hypothetical future society).
The crowdsourced part is important. Technically a government could buy all the stuff for its citizens, but that is very inefficient for certain things as the knowledge of what is needed is local to the citizens. Additionally in the way we currently structure our governments there is too little room for experimentation: A government that would regularly try multiple different approaches to serve the same market simultaneously would be hammered by its citizens in a democracy for "wasting money", even though that is exactly what a healthy free market would do (bankruptcies and failed projects are a feature of free markets).
Coming back to the question what the percentage should be: it is partially a question of which markets should be publicly funded and which shouldn't. Clearly things like defense, the fire department and the justice system should never be privately funded. I believe health care shouldn't either, btw. The point here is that we have reasonable grounds on which to make the decision, namely what amount of free market capitalism can be allowed in a market without detrimental effects to society (what are essential services and whatnot) or rather which way of funding produces the most desirable results for society.
Determining which market needs what amount of resources is much harder (technically you could spend 99% of GDP on defense), but also part of it. How much investment do these publicly funded markets need? Is that even the right question? There is clearly some minimum to facilitate bare functioning, but what is a reasonable amount/service level? I would say this is a matter of experts making decisions based on a democratic mandate.
The other angle would be to think about what amount of personal spending money is beneficial to society: above what amount do people start spending it mostly on things they don't need and that don't make their lives or technology better? This is tricky, because it is highly intertwined with what services are provided publicly and because it's hard to come up with solid reasoning if there is an upper limit to what more spending does for (technological) progress. There is probably some value in stimulating the caviar and yacht markets, but I'd guess not that much.
All in all, it is tricky. Given all the above it may actually be a lot better if the tax rate was 80% for low earners and 95% for high earners. It may also be better if it was 20% and 35%. I don't think the latter is true, but I can't say with certainty that the former is either.
Again, it is a hard question.