I would completely revamp the way staterooms, common space, and utility spaces are calculated.
Currently common space is simply part of staterooms. But this breaks down when you want to create different sized rooms, suites, cabins, bunk rooms, etc...
Personally I would make lounges a factor of maximum capacity (total people) instead of "about 1 ton out of a 4 ton stateroom".
Mostly this becomes a problem when trying to create deck plans. A 4 ton stateroom and a 4 ton med bay, library, shop, etc... are not interchangeable. Some version of Traveller (Like GURPS and T20) use "about half" when it comes to stateroom actual size vs. stateroom design size. This means most GURPS and T20 staterooms are actually 2 ton rooms (4 squares) with 2 tons of "common space" distributed throughout the ship.
If I were to start from scratch, I would give specific sizes for different staterooms; basic, standard, high, suites. Basic for instance would not have it's own fresher unit and be just large enough for bunks, a desk and small closet. That would be a 2 ton space.
Suites could go up to any size you want, 8 tons or even higher.
Common space like lounges, galleys, etc... would be based on capacity. You count up the total number of passengers per stateroom, give 2-4 tons per person, and that determines your common space... (which includes circulation space like corridors, stairs, lifts, etc...)
Then I would size life support based on capacity as well. 10 people = 1 ton, 100 people = 8 tons or something like that.
This would also fix the problem with bunkrooms. Currently bunkrooms are technically a group of staterooms without walls. The total occupancy for bunkrooms work the same as individual staterooms. If you've served in the military before, you'll know a bunkroom has many more racks than a traveller bunk room. You simply count the number of bodies and factor that number into the size of the life support space.
It's more work, but for designer/architect/deck plan geeks like me, I like those kind of specifics.
In the current system, where is main life support? Where is the water purification systems? LOX supply/scrubber/convertors? If a ship takes internal damage would it make since for a ships mechanic to scramble and fix life support if it takes a hit? Currently life support is generically tied to the power plant.. "if you lose your power plant, or run out of fuel, you die from lack of air" etc...
Anyway, these are a couple things I would change.
4 tons per stateroom as a "catch all" space just seems a little to generalize for me.
BTW... GURPS Traveller actually takes some of this into consideration with "utility" spaces, covering life support and artificial gravity machinery.