Okay, this one is going to seem strange to some people.
To me, a lot of the trouble that people have where it comes to "designing their world" is that they don't know what to design. The default is to design things that are very small - buildings, streets, dungeon hallways and rooms, etc. These could be termed 'microdesign' in that they serve to apply to the immediate needs of the party, in terms of what is five feet to their right and fifteen feet to their left. Useful, obviously. But apart from the immediate, these things don't help much where the party decides to march off in a direction, say a large open landscape where they're going to see two thousand acres in a day. You can design one small part of one acre, but you can't really get a sense for the scale.
What is needed, then, is 'macrodesign.' Here, I think, the game and most of the players are notably lacking, because they just don't know how to approach the problem.
To try and sketch out a method, I'm going to return to an image I once used to describe my trade system: the map of Hothior, from the old game Divine Right!
Ah, so familiar, so conveniently neutral. I am going to use this to demonstrate a system which really works better on the maps on my world, but for the general audience I think it would be best to simplify that so that anyone can apply this.
That does mean I will need to add some information to the above to make my system work. To start with, I need to give a population figure to each of the above cities, Port Lork, Tadafat, Lapspell and Farnot. I also need to establish a population for the region.
If the hexes are 20 miles across, the above region is about the same size as modern day Bosnia (without Hercegovina). Recognizing that there really ought to be more than four cities in an area this size, I'm going to identify the population at 2,000 per hex (there are 50 hexes, counting all those on the water as a full hex). Total, 100,000.
For city populations, we'll say Port Lork has 8,000 people; Lapspell, 6,000; Tadafat, 4,000 and Farnot, 2,000.
I'm introducing these numbers so that you, in your world, can take the probable size of cities that already exist. Obviously, the population isn't going to be distributed evenly throughout the countryside. More importantly, neither is the infrastructure ... and what I mean to do is produce some infrastructure numbers we can work with.
Let's take the total value of the cities (20,000) and divide it into the total population. From that, we can presume that each city influences 5x its base population: Port Lork, 40,000; Lapspell, 30,000; etc.
Let's further divide these bigger numbers by 259 (persons per square mile). We'll call this the BASE Infrastructure Number. For Port Lork it is 155; for Lapspell, 116; for Tadafat, 77 and for Farnot, 39. Let me add those numbers to the map just for convenience. In addition, I'll color code what amount is coming from which centre: