wandering a bit off topic...
A lot of the problem is that different sets of shipping paradigms exist based upon technological era. The "hard numbers" really only exist in reliable nature for the last 50 years or so.
If you assume paradigms similar to Earth, circa 1600 CE, long range trade is usually speculative, due to uncertainty of completion, and the ability to get local reproductions faster than foreign ORDERS. Foreign goods sell well, when available, as status items. Some stable links exist, but the actual parcels are speculative, not that the risk is high, just that the prices have stabilized and increase from source. Salt, Silk, Pepper, Cinnamon, Saffron: all rise in price as you move from the origination point.
Circa 100 CE, things are a little different. Trans-Mediteranean trade is known, and little is known about the overall flows, but a little bit of documentation suggests strongly a multiple-speculation process. A runs between AA and BB, B buys a mix of BB and AA materials, the later from A; and ships to CC, where C buys a mix of AA, BB, and CC materials, some of which are from B, and runs them to DD, where they are sold to local consumption; some are diverted at each step. Silk travels this way. Known, and traded, but 5th-50th hand. The source point is essentially unknown to even the merchants carrying it to Rome.
Perhaps the source is unknown, perhaps not cared. All that matters to the trader is knowing that he'll get a better price at the other end, and make a profit.
The various methods out there make various abstractions and assumptions; it is a contentious topic. (Heck, to be honest, it starts flame wars!) The fundamental assumptions and abstractions are problematic.
So, if the system works for you, and matches your base assumptions, enjoy!
I've not used QuITS, but I think it valid in a very generic way... Rob's not divulged the baseline data he's extrapolated from, if any.
Several factors to consider:
Several persons (including some SciFi Authors) imply that most multi-stellar polities exist not for economic, but for sociopolitical and socioreligious reasons. Trade is primarily focussed on scant luxuries, and possibly drastic surpluses or major shortfalls; day-to-day operation is almost entirely local. (Webber; Cole & Bunch; possibly also Wheedon) Note that specialized goods still get moved. Cole and Bunch's Sten series assumes a hydraulic despotism of AM2, but most of the other empires are small, and religious or sociopolitical zealot-ish regimes.
Others take the view that most worlds will not be self-sufficient, and thus must trade to survive. (GT:FT; SJ Ross; I think AD Foster's Commonwealth falls here also.)
Still others imply certain tech is localized, even though it could, in theory, be built elsewhere , but are not for various reasons; most "Staple" goods are not traded, but many desired goods are sold in ways that resist local replication, and the manufacture processes are kept tightly controlled; these goods might be essential, but they are not "Day-to-day" trading environments. (Bujold, McCaffree, possibly Cole & Bunch) These also seem to assume a much different set of tech paradigms that Traveller, in that the scale is more a bush, and one travels down various paths. (Great for literature, lousy for GM's sanity.)
Also important: How much travel is there?
SJR points out in the above a good point: As demand increases, Ship Size increases, rather than hulls, unless some choke-point exists.
Couple restricted hulls with essential trade, and you have huge numbers of small ships...
Couple unrestricted hulls with essential trade, and you get HUGE trade volumes in a few HUGE ships...
Unrestricted hulls and limited need, and you wind up with Firefly. Huge navy ships and commercial liners, loads of small tramps filling in the niche markets... Most of the worlds are habitable, and survivable without trade, and trade is for comforts, not needs.
