As described, your tax system will be difficult to implement
The problem with taxing by proportion of income is that it will be very hard to enforce without a ridiculous amount of literate manhours.
Every single individual will somehow have to show how much they have earned in the previous tax period, which will be impossible unless they are able to keep accounting records (and thus be able to read), or they are able to point to a warehouse of produce and say that is how much they earned.
The actual division at that point would be much more straightforward. Whilst complex mathematics would be beyond most people, the basics of counting and giving one share in every ten would be doable.
This assumes that the individuals are all honest, which is highly unlikely. Thus you would need literate taxmen who are able to suitably appraise the income of individuals and levy the appropriate tax.
Instead of taxing by variable and easily falsifiable values, I suggest you tax based on simple, discrete items.
A common tax in Early Medieval England was the Geld. This was a flat rate based on the amount of land owned by an individual. Land was synonymous with Wealth, as it could be directly used to produce resources or you could let it out for Rent.
A second common tax was Custom Tax, Applicable against all cargo that moved through a specific area, such as through a city's gates, a checkpoint on a road, or down a canal or river. This worked well, as you could claim a portion of the goods then and there rather than having to determine tax based on a nebulous concept of "income". Another way this worked is to charge a monetary tax based on the value of the goods. (One particular case is the Sound Dues, where any ship passing through the Sound was required to state the value of the goods carried and offer them for inspection, under the proviso that the crown was allowed to buy the product at that rate instead of demand a 1-2% tax on its value.)
There was also many other fees that weren't technically taxes, but were in practice. For example, peasants may be required to pay a proportion of their grain in order to have it turned to flour at the mill owned by their lord, whilst simultaneously being banned from milling the grain themselves.
Thus you will never need to audit suspect individuals as the taxes claimed are all based on obvious circumstances. Sure, some wealthier people may get taxed less than a poorer person, but that's only considered unfair by our modern viewpoint where our finances are much more easily tracked. If someone is moving a lot of resources around (which they'd need to do to get "value" out of it), you'll get your fair share.