Question 1: When eating at a table the 90% you saw was representing that the food only fills 90% of the normal amount of hunger filled.
Question 2: the first number (f.x. 90%) represents the amount of the hunger level of the bar you're at at the moment.(There's 5 "levels" of hunger: Starving, very hungry, hungry, full and overstuffed). the second number tells you the total hunger % amount of the hunger bar which is filled.
Question 3: fishing is a good start (you need the fishing skill to do the things explained here.) make a fishing pole form 2 branches, a spindly taproot (can only be found if you have foraging), plant fibres (cna be gathered from hemp or flax as a byproduct laying on the ground when harvesting the crop) and yarn, (wool spun on a spinning weel, can be made if you have animal husbandry and cloth making) and a bone hook from a bone (Kill a rabbit, butcher it and craft the hook from the bone.) Also, you need to find some bait or lure; earth worms can be found in forest terrain by digging (requires pottery), rock lobsters, (chip a stone from a boulder and make it), pinecone plugs (can be made from a fir tree pinecone and 1 piece of birch tree bark) or feather flies ( Can be made from 1 chicken feather and a block of wood) are all common lures/baits. When you have the two branches, just make a fishing pole in the crafting menu. then, left-click on your string and right-click on the fishing pole, then add the bone hook in the same way, and then again the bait/lure by using same method. Now that you've got your fishing pole, equip it and find a place where you see fish jumping around in the water. in the adventure menu, choose the "fish" command and click on a place where you see fish jumping in the water, then your Hearthling will start fishing (you can fish from a boat.) Once you start fishing, wait a bit, and you might catch something. If your Hearthling stops fishing without you telling him/her to do it, one of the fishing pole's parts are probaly broken. Then you have to go find soem new materials.
Question 4: houses can be used for building cupboards, which is a big container, but its also useful because things inside the house wont decay.
Hope i helped.
