The 1.5m x 1.5m x 3m squares, implies that every bit of space is a that shape. I would say its not the case, (well at least it my humble designs) hulls are various shapes so I allow extra 'squares' for bits pinched off by lower parts of the hull and vice versa.. So its worth allowing extra or less for different heights of space in hull. The stuff I do always keeps an eye on the external shape of the hull as design that too! Though I fill out some sections with 'black' space ala the older plans in White Dwarf etc... that can allow for the pinched off sections for effectively.!
Things to bear in mind:
Raising or lowering the height of certain sections.
I.E. fuel in wings (1.5m) High or Cargo deck ceilings at double the height.
Combining sections ie sickbays or staterooms is a nice touch.
Adding in rooms etc from tonnage assigned to other things (ie lounges, med bays, galleys, ready rooms etc)
Its worth adding large fixtures like tables, sofas etc...adds a nice touch.
Small Craft being small craft, get double the squares as they are thinner... (I think I got that right?)
I think ultimately as long as the plan looks right and doesn't take the michael thats ok with me. My head would melt if I worried too much about the internal volume...
