Thanks! My guestimates were between 1/1200 and 1/1500, based on the fact that the Ad Astra/Mega Miniatures Free Trader was 1/1000th. My math kept trying to tell me that the scale was 1/670th, which I knew wasn't right - must be using the wrong formula.
That ratio is very handy to know - that means you can do a scratchbuilt model up, then do a volume test on it to discover its "true" tonnage, without having to worry about it before hand.