We move everything to the V-berth when on passage
and use the pilot berth. It changes the weight balance of the boat but in our case, in a good way.
There are lots of reference materials for the X/Y questions you have, including Beth's great book. However, as far as diesel
goes, think more about range and gallons per hour because, depending on a few factors, two boats of the same type might have very different ranges even though they have the same gallon storage. For example, Prop size and type can make a big difference.
is really a question of how much you need per person per day plus a reserve. We carry 150 gallons but I know many circumnavigators that carry half as much, don't have water makers, and say they get along fine. We are planning on 1 gallon per person per day. For example (and not to be taken seriously), if you take the longest trip on the milk run, which is about 2900 nautical miles on the rhumb line, about 30 days at four knots, then add 50% for reserve, for two people you would need about 90 gallons of water (without water maker).
z cubic feet: take only as much personal stuff (clothes and all) from land as you can in a large duffle per person - you won't need anything else, and the rest of the space on your boat is for food
and spare parts
On our 38 foot boat, we have enough food
stored to last two of us 4 months, beer
for 3 months, liquor for the year, and we don't have that much food storage at all! When we got rid of all the crap we brought with us from land life (honestly thinking we would need is a black and decker air pump), we now have more room for spares than we need.