What is an optimal minimum and maximum route distance for ships?

[GoldMine]

I've set min/max to 81-250 manhattan distance, this is according to may limited gameplay so far solid for ships (but I'm not sure)

Problem is when I map for coastal/river industries or towns I get very small number of candidate routes on 1024x1024 map, plenty of routes are skipped due to being out of distance range.
It's easy to make distance range larger, but question is by how much until a route becomes non profitable or until ROI (Return On Investment) is too small to be worthwhile.

I'm sure openttd veternans can give some hints.

[GoldMine]

OK, this was actually stupid question.
The optimal distance depends on cargo value and profit gain, how much one considers optimal is pretty much subjective, what matters is to select the most profitable distance/cargo payment.