To summarise just a simple question:
I understand that placing a TP which does not contribute to longest route does not get long route bonus
I understand that placing a TP which actually closes a long route into a loop can actually degrade the long route bonus
Within the above bounds, if I have an asteriod which has nothing to do with the long route, is it still worth placing a TP on it? Or should I only build TPs on long route?
Plus, did I understand that, within the non-long-route bounds, it does not matter how big/developed the asteroid is or isn't or where it is, in all cases TP bonus is identical? But if it's far from your capital, you won't actually get so much money out of it?
The longest route bonus is figured by taking any two planets you own and finding the shortest trade route between them (if one exists). Repeat for all planet pairs. The longest of these shortest routes is your "longest route." Confusion over the term "longest route" comes from the fact that there are certainly longer routes visible, but there aren't any longer "optimally short" paths between any two worlds than the one calculated.
The bonus given by the longest route affects all trade ports, whether or not they are along the longest route itself. Thus, it is generally worth it to place as many trade ports as you can, except in the case where placing a trade port creates a shorter path between the two planets that currently are on the ends of your "longest route." It gets a bit complicated though.. to really figure whether it is worth it or not, you have to weigh the slight loss to all trade ports from a shorter longest route compared to the increased income of having more trade ports.
My general rule is to first place trade ports along my planned longest route and get that established. Next, I fill in any worlds that don't shorten the longest route. At that point, once all logistic slots are used on all those worlds, I consider filling in any remaining planets.. considering that you also have to build research structures it makes sense to use so called loop worlds for that purpose.
Lastly, consider building starbases on uncolonizable planets for trade only if it will extend your longest network (that is, it will connect planets in such a way that one pair of planets has a longer shortest path than your current longest path). The investment of a starbase is vary rarely worth the expense if it is only to be used for trade, even when it does extend your network. The only time I'd consider putting a trade dock on a starbase (in a competetive game) is if I built it to protect a front line that has since moved outward and I have extra cash, have built all other trade ports, and the starbase still has an empty slot. In all likelyhood I'm already winning such a game so they don't ever factor into gameplay in a meaningful way.