But dwarves would drink a different amount if they were in a desert than if they were in a temperate broadleaf forest. Also the dwarf might spill a lot of the water when he drinks it, which would throw off your caulculation.
I like to think of the tiles as 7' squares. This way the height of water is in feet, and dwarves have to swim in water over 3' high. Which, again, kind of makes sense.
What doesn't make sense is that a wagon traveling up a ramp is going up at an exact 45 degree angle. Which is quite difficult to do in reality. But we can probably just ignore that.
Anyway, as to the dining:
I always have just one dining hall. If someone needs a personal dining room, I just assign them one of the seating places. I typically have 1/5 the population in tables, but I like to stick 40-50 in there as soon as possible just so I don't have to think about it later.
Always smooth it first, keep smoothing elsewhere until your engraver is legendary, and then have him engrave the walls and floor. Install only masterwork furniture, and if the dining room isn't legendary start changing the regular masterwork stone furniture with masterwork obsidian or metal furniture.
The doors are always a masterwork obsidian unless I for some reason have surplus high-quality metal doors.
Also, side note, my dwarves share a communal bedroom. Each bed is assigned to a dwarf. I find that I need about 2 beds for every 3 dwarves because they get married and dwarfbang all day in the same bed. So if I'm planning for 200 dwarves, I build about 135 beds. The remaining space in the flophouse contains 3 each of masterwork obsidian or high quality metal cabinets, armor stands, weapon racks, and coffers (which are not assigned to anyone). This way each of my dwarves believes he owns 12 pieces of furniture (plus a bed). I find that my bedrooms hover just under Royal. In my current fortress I have an artifact olivine cabinet worth 30,000 monies, which helped my bedroom immensely.
My nobles gather together in a single office crammed with traps made from masterwork steel weapons and masterwork mechanisms. I assume it helps room value but I'm not sure.