The math is correct, fair, and working as the devs intended. However, it is somewhat non-intuitive at first. The best (short) explanation I've found is by SageWon:
Quoting SageWon,
reply 4
"is is there ever a situation where destroying a trade port will actually be worth more to your income than keeping it there?"
Yeah Garv, here is nice (extreme) illustrated example of just that.
But remember, in Entrenchment, trade routes don't automatically jump over UCGWs, like they do in vanilla Sins. You have to build a starbase with the trade upgrade there, for the route to extend thru them. (UCGW=UnColonizeable Gravity Wells)
That SoaSE wiki site also has a good explanation of vanilla Sins trade.
The illustration on the wiki was screwed up a bit. I have fixed the image reference, so the pictures should make sense with the text now.
fair enough... so you're saying i have to strategically place my trade ports? okay, but there are times where the optimum trade route line will stop for no reason, there will be another gravity well or two it could easily expnd to, but will not, and i cant figure out why?
(and yes, i figured Starbases with trade post upgrade would be needed, and rest assured all UCGW's had starbases in them)
on the other hand, i plaed a game on a circular, symmetrical map, and as i expanded, the OTR (Optimum Trade Route) continuously expanded and ran along my front lines, (considering i was expanding to my left and not forwards) as, eventually, i had taken over half the system and so logically the longest route ran across the system... however, i also find that the OTR likes straight lines, and wont take a turn, rather, will prefer to keep heading in one direction if its possible...