So, you are replacing pots with PLC and analog outputs?
If that's the case, then you should be able to calculate everything in engineering units, including the trim values.
I would have a scale factor for each drive that will turn the master reference units into drive speed command units. This will allow gear changes on individual lines to be easily dealt with.
For the trim values, you may just want to use a percentage. For example, the first conveyor trim factor might be 1.00, the next conveyor might be 1.05, and so on.
You could also apply multiple scale factors to each successive conveyors, so that, for example, conveyor two runs at 1.05 times the master, and conveyor three runs at 1.03 times the speed of conveyor 2. Doing that would ensure that each successive conveyor always changes with any adjustment to upstream conveyors.
The above is just an idea about how to program this, but perhaps you are needing info about the hardware to choose?
Paul