Yeah I understand that. But given the group of people on this board primarily devoted to Traveller, I would imagine we have the capability of building some form of UWP that represents import/export GWP/RU factors in a manner that all players would have a better idea how the systems work. ...
Well, we probably could, but we aren't building a UWP. We're taking the UWP and the rules that the game gives us and using those as tools. The game's rules are by their nature imprecise and intended to give the game master a lot of freedom for his imagination.
There are only three factors being looked at in the original game: a world is agricultural or nonagricultural - or in the middle; a world is industrial or nonindustrial - or in the middle; a world is rich or poor - or in the middle. Then Merchant Prince adds a few categories like Hi-pop/Lo-pop, Vacuum, Desert, Water, and so forth. However, our attention for the moment is on agriculture.
In this case, we're dealing with a rule that says an earthlike world with a population in a certain range is an agricultural world. That does not mean the world is a breadbasket feeding the subsector. It means, in the game of supply-and-demand, that they have spare supply of foodstuffs to sell to others and bring in some currency. They have more than they need, ergo excess supply, ergo what they have is available at a lower price. They do not have spare supply of industrial output, so no bargains there; in fact, if the pop is low enough, their industrial output costs
more than average 'cause the demand is greater than supply - which makes them a nice market when you sail in with your cargo of industrials. But as far as the Ag code is concerned, it means, "Here is a world that produces more than they need, so you can probably buy it cheaper than on other worlds."
Can you calculate what the available extra is? Sure, within a broad range. Easiest way to do that is to calculate what the population needs, then say there's more than that. How much more? Well, however much you want, depending on the tech level. You can built a robot around TL8. You can build a rather good basic robot around TL9. It does not require great intelligence to be a robot tractor pulling a hoe, or a robot harvester: bot brain needs to be smart enough to follow a programmed pattern and to stop and call for help if anything looks odd - or if a sizeable object gets in its way, 'cause it wouldn't due to accidentally run someone down and you don't want your hoe damaged because some prankster plopped a log in your field. Nor does it cost an incredible lot - you make up the cost in 4 or 5 years from what you save on farmhand salaries, pure profit after that. Still need someone to supervise the things, but basically you've cut your farm workforce to a small fraction of what you'd need today. Imperium's not big on bots but it is big on profit, and nobody's gonna fret about a bot out in an open field away from everyone.
So, from about TL9 forward, you could conceivably have every square foot of arable land on the planet under till and have a local population of as little as a few hundred thousand - with the bulk of those being bot techs doing annual maintenance and the occasional repair. The only thing that stops you from doing that is there just isn't a market for that much food - unless you happened to be a parsec away from a high-population vacuum world, in which case you could get very rich doing things that way.
Now let's look at some numbers. Pop code 5 to 7: anything from 100,000 to 99,999,999 souls. Borrowing on the Book 3 Food and Overhead rules, about a kilogram per person per day: 36,500 to about 36,500,000 tons is consumed locally every year. Keep in mind, our TOP population figures are comparable with the population of Egypt or Mexico. On a size 8 world with 70% water, that means there's still huge areas of the world that have never seen a living soul. Anyway, anything over that consumption figure is overproduction - some desperate producer who can't find a buyer and will lose money unless someone arrives to take it off his hands. (We are not messing with wastage and loss, folks, live with it.) How much more is there?
Well, that's where the devil hides in the details. If there's not much of an export market, not much more. If there's a big demand out there, lots and lots more. Excess results in lower prices, which is a disincentive to expanding production, so the agricultural output of the world is going to expand until it reaches the limit of the available market,
including exports. On a world with an E port and little trade, it might be no more than 20% in a good year, and the farmers who are late to market will be cursing. On a world with an A port and lots of hungry neighbors (and no nearby competition), it'll be as much as the neighbors can eat, right up to the point where the last arable acre is in production - under some bot. Usually somewhere between the two, but the point is that at TL9 it'll be demand driving agri production, not population - except that more and more of the world's carbon-based pop is working in building and servicing bots, but we don't have rules for that (and some of that is roboticized too).
Now, if your world is tech level 8 or lower, then we can look up some historical figures and put those to work, but it's still only an approximate.