OK, this is a fun question. And I'll try to give an overview (though, very much amateur enthusiast historian here - I'm not a "proper" academic medievalist!)
The nice neat idea is that in the feudal system in theory the overlord (king) owns everything. Lesser nobles (the "tenants in chief") hold land "from" the overlord or hold land from the tenants in chief. Some land is directly owned by the overlord, but he won't actual administer any of that himself - he'll appoint administrators and commanders (shire-reeves, castellans, etc.).
In the Domesday book, 17% of land is royal land, 190 tenants-in-chief hold 54%, and 26% belongs to bishops and abbots.
So, if you want to keep yourself sane, you could just assume for a "typical" feudal area, about a fifth = royal, a quarter = various bishops and abbots, and the rest (over half) is barons, earls, etc.
But a lot of the fun (including a lot of potential for stories and interesting settings) is in the detail. So, here is some other stuff you might want to play with:
- An overlord can demand that a neighbouring magnate acknowledge him as an overlord without ever owning any actual land in that magnate's territory. E.g. the "I'm the King of France, you're the Duke of Brittany/Anjou/wherever - bow down to me so I can demand you send me soldiers and taxes in future" requires no change in local land administration.
- Cities are becoming important, and their owners generally let the city's grandees "buy" a degree of self-government in return for fixed rents or money or service. This is utterly unimportant in, say, Scotland where there are no settlements with a population above 2,000, but is really important in, say, Flanders and Champagne, where there are big cities. A town might negotiate a "charter" where it administers its own affairs, appoints a mayor, administers "low justice" (not murder etc.) in return for a payment of, say 1,000 "soldiers" (i.e., armed citizens) for a month per year, or enough money to pay for 100 soldiers for whole year. At the start of the 13th C London gets a charter; by 1220 there are dozens of towns in England who also have charters, though usually for much lower fees than the example above. This seems kinda weird, because it means the owner (usually the king, but it could be a bishop or noble) is giving up control of economically powerful areas, but it's about simplicity - the king knows nothing about running trade, but certainly wants people to promise him predictable revenues or soldiers. Tensions between militarily-minded feudal lords and "free" commercially-minded cities make for good dynamics in sagas.
- Royal lands may end up being the more strategically significant. Example, when William Rufus expanded the borders of England north to Carlisle, he made sure that the lands around Penrith (i.e. the lands on strategically important road from Lancaster to Carlisle) remained royal lands, but the impoverished and windswept hills off to the east he essentially subcontracted to one of his strongmen - saying in effect: look, you go subdue this unimportant area for me and you can be the Earl of Egremont, but I'm keeping control of this vital road.
- Rights and privileges could overlap and get messy. The system was not neat. And this is fun for creating stories, either small or grand....
As a petty example, maybe the king of England has "given" this bit of land to an abbey but has still designated it as a royal hunting forest so only he can hunt there, the abbey has given rights to a neighbouring village to graze their pigs on the land, and now the magi want to gather the acorns that fall from the magic oak - lots of people are jostling in that space, with the magi annoyed that the pigs keep eating the acorns, the villagers are paying money to the abbey for the right to do this, but the abbey are actually considering cutting down the oak (which they many or may not be allowed to do, based on forest law in England). That is probably WAY more complicated than you want to go, but it does give an idea of how complicated this can get.
On a large scale, here is a real thing: The King of Scotland is also the Earl of Huntingdon. So, when a new King of England is crowned the King of Scotland goes and does homage to him. In the King of Scotland's head (and as he tells his own underlings) he is only doing homage for Huntingdon - he is still an independent ruler in Scotland. The King of England, meanwhile, happily crows to his minions about how he is superior to the King of Scotland who is, of course, one of his vassals. This tension can fester for ages until eventually one English king decides to push the point and actually exercise what he thinks are his rights as the Scottish kingdom's overlord (at which point a big war breaks out, and in later years terrible movies will be made with Mel Gibson inexplicably painting half his face blue). You may not want to use that exactly, but you can se how ambiguity in land holding can give you great tensions in a saga, with rivalries between nobles (all of whom think they are right) embroiling a covenant and leading to war.
Hope that helps!