After corporation, my numbers of Freight steadily increase until the SS landing. Typically, I'll have 10-50 freight in transit, in position, or ready to deliver (this may well bump 200 in a large post-stealth, peaceful world). Your objective should be to keep from getting overwhelmed.
Personally, I don't like taking notes, so I think its best just to develop a system geared to making specific freight trade get to the best cities. I use patterns of sleep and fortification to determine what a specific freight should do. For example, if I get a silver caravan sent across the world, and it is waiting outside it's destination city for delivery, but it is not needed on this turn because the science would be wasted (a very common scenario every turn after flight)... I fortify it just outside the city. A few turns later, when I scan teh board for a caravan to deliver to get that science up to the 99% mark again, any fortified caravans are readily identified. Double check before you blindly deliver, though, because sometimes the demanded commodity will change.
It is important to get demanded cargo to the most distant, proper city that is using all available trade squares. If you do this, many of your freight will deliver over 2,000 (and some can peak over 4,000).... and that is for a single freight after Flight. Undemanded freight is more like 400 to 800. Money is no object after flight, and almost all improvements can be RB'd or PRB'd. There is almost no need for science production (maybe 15 or 20 beakers... a couple scientists will do), and 70% taxes can be set, yet still make one advance per turn.
Set your taxes down to 30%, science up to 40%... and you should have two advances per turn, particularly if you keep gifting the purple civ with tech (assuming your power is supreme). Keep the purple beaten down and isolated from everyone else... for that matter, keep everyone isolated as much as possible!