When a convoy needs 1 month to go to destination to unload cp units, it needs another 1 month to return empty to reload. So, it needs 2 months to carry cp units. That means it carries 1/2 cp units per month, while it delivers cp units each time. If cp/month is the consumption rate of destination industry the sufficiency in storage should be 2*cp and not just cp, as it might be explained in simuconf.tab. If there were n convoys, industry should be ok with 2*cp/n. So, as it seems it is, the routine hurts the effectiveness of 1 convoy or reduces the production to half, is exact with 2 convoys, and bonus 3+ convoys without preventing overstocking. You can take a look at "Stock limit" paragraph in 12th page of my notes in https://drive.google.com/file/d/14TGL_3VeJe_k0U_WteQWR2jzF4v6CZFt/view?usp=sharing
Can you clarify what you meany by CP units here? Thank you.
"cp" (from capacity) units might be crates piece goods or bulk goods or whatever a vehicle carries. A vehicle may have capacity 4 but schedule 50% load and carry only 2 each time. In that case cp=2. If to reach destination needs 1 month, the go and return journey needs 2 months. So, this vehicle transports 1 crate per month.
The point is that, concerning a specific order, there is always an empty return back trip.
If the game estimates duration of trip by actual arrivals, there is no problem but to the description in simuconf.tab. But if it estimates duration by ratio of distance to some velocity then, i suggest to assume double the distance (because of the empty returning trip) so as not to harm lines with 1 convoy only.
From the point of industry that needs goods, if there is only 1 convoy and she needs 4crates/month from a distance 1 month away, she expects deliveries every 2 months because that is the time a convoy needs to return to her. So, if the single convoy departures from source industry right now, it is necessary for it to carry 8 crates.
Thank you for your attention.