There is a somewhat active discussion over on COTI about the T5 rules. And I may be misinterpreting something. I thought that spec trade was handled outside of bulk cargo: bulk cargo is what is sitting on the docks, so to speak, and someone is willing to pay the Cr1000/ton to ship. You have the major, minor & incidental lots, basically sealed containers. Really there to fill up the remaining few dTons of storage if you want a full cargo hold, or can't wait for the weekly roll on the spec trade.
Spec trade was rolled weekly for what it was and the tonnage, at least in the earlier editions. Still doing that - the software in theory adds a week every time you do a search for cargo and passengers (this is configurable by version but defaults to a week). So each time you click that button, I go through all the systems within jump of your ship, generate the passengers and three levels of bulk freight for each system, then generate a random speculative cargo. It is that last cargo I cannot find the tonnage for, UNLESS we use the freight guide which is:
(Flux x Pop) x (total TCs + 1) for a specific list of trade codes. I think I'll just go with that.