Celebrim
Legend
I used those two rules to come up with a basic tax system. Most nobles want to get as much as they can out of their peasants. So they set the taxation rate at a level where the peasants can live at a subsistence level. This is common, but not monolithic.
Common, true, but typically not monolithic.
England was restive and independent, and tended to have low taxes.
The western end of the continent tended to be slightly higher taxes, while the eastern end had slightly higher than that.
The worst cases were Eastern Europe in the 19th century, were taxes rose to 80% of peasant income leaving the peasants in essentially perpetual starvation.
While nobles wanted to maximize taxes, if you set them too high you ended up with poor peasants or dead peasants. Some time before that, you had angry peasants who can and did go into revolt, become bandits, complain to the bishop or the king, and otherwise become a hassle. If you set them lower, your regime in the long run tended to be more successful - the peasants would make investments (build apiaries, dove coots, clear and farm more land, etc.) and become wealthier (letting you charge more taxes), and the peasants would tend to not die off as quickly following a bad harvest (because they had savings) meaning you had more people to tax over time. However, if you set them too low, then you couldn't field an army and your neighbor would come and take your stuff (since Knights were often little more than bandits with a license).
Cost of living depends on both your expectations regarding your lifestyle (you eat meat 7 times a week, you have more than two changes of clothing), and the maintenance on your property (you have a horse to feed, taxes to pay, a roof to thatch, and armor that has to be cleaned with vinegar and tumbled in a barrel of fine sand and then coated in rapeseed oil on a regular basis if it isn't to become rusty). In general, for PC's, I tend to make cost of living a function of character level, and then add sundry costs if they decide to purchase a steed, etc. For most campaigns this is mostly ignored, but if you run a dynastic campaign where the focus of play is building up institutions it becomes a huge concern and time sink.