So, out of interest, how would this work for regular people? I mean, you don't have enough space in your backyard to grow all that much, I'm willing to bet. I certainly wouldn't. I remember you saying you could apply it to things outside of agriculture. Would that essentially mean not much change for people? Just continue your job but a portion of everything that you make goes to the government?
Those are some very important questions, in need of very detailed answers. Which I will answer, when I'm done designing a metal-as-fuck banner for my kingdom.
Alright, I'll throw something out on principle. Essentially, it wouldn't have to work much differently from a modern economy. The only real difference between medieval industry and modern industry is that we have much more precise ways of calculating and demonstrating relative monetary value. The key different element of Feudalism, and I'm probably pulling this out of my ass, is that while it uses an income tax like most modern government-regulated economies, it's drawn up backwards. Instead of forking over a percentage of whatever value you make, you as a fief within the country (and if we're taking this beyond agriculture it's not even tied to land, just citizenship and a locality) must be able to fork over a certain amount of value, and you get to keep whatever you earn beyond that.
In exchange for paying a set rate, you get all your normal civic services, like roads, law enforcement, and military protection. But because the foundation of the government is on the entire working class paying its due, it's more or less against the law to be poor or unemployed. And if you break that fundamental law, then God knows an autocratic military state knows how to make jobs for people. By extension, that pretty much means that you can't emigrate or trade outside the nation without permit or take up arms or really anything that the government does, because that's what it's there for. It would kind of silly otherwise, to be taking up all that value when you're doing the government's job.
And for those of you getting worried, remember that I'm your King. I would never ask anyone to do something I wouldn't do, and while I have essentially no pride, ethics, standards, or squeamishness, I don't like working very hard. History may well be on my side in that one - my economics professor was (claimed to be) writing a book about medieval economics. He said he found all kinds of evidence that the average peasant only worked about five hours a day. That's what all the fairs and tourneys and markets and taverns were for, everyone was partying all the time. (And it's a very well documented but rarely mentioned fact that most of humanity for most of history was drunk off its collective ass.)