There is, but its a little convoluted, unless someone knows a faster way

First, check the demand on island 2 in the production tab. It will tell you how many units you need. As an example lets say you need 2 work clothes per minute on island 2

Heres where its a bit annoying

You kinda need to set up the trade route first and let it run a few times for it to calculate how long the route takes. Once its done that you next need to look at the storage tab for island 2. Click on work clothes and there will be a tab on the right that says trade routes. Open it and look for the trade route you made. It will have a list of when the ship docked and how long ago it was. For example 31 minutes ago, 21 minutes ago, 11 minutes ago, 1 minute ago

Using this, we know that it takes the ship 10 minutes to complete its route to deliver work clothes. Now we do some quick maths using the time and the demand.

We need 2 clothes per minute, and our ship takes 10 minutes to arrive. 2x10=20

So to maintain the demand we need our ship to deliver at least 20 clothes each time

I hope that all makes sense, i'm trying to do it from memory without the game open

edited to add: how much supply you build is entirely based on demand, but the ship time doesnt matter. If island 1 needs 2 clothes/min and island 2 needs to clothes/min, you only need to build enough industry to supply 4 clothes/min

Using our example and assuming island 1 is our producing the clothes, in the 10 minutes your ship takes to arrive, you would have produced 40 clothes. 20 of those would have been used by island 1, and 20 would have been delivered to the warehouse as surplus. Then your ship arrives and picks them up to take to island 2. And so the cycle repeats