Okay I think I have all the relevant petrochem researches

The main distillation chain is crude -> medium -> light, which yields heavy, diesel, naphtha and fuel gas
Heavy can be turned into fuel gas + either naphtha or disesl in exact same amounts
Naphtha can be converted into gas + same amount of diesel as naphtha input.
Diesel also can be converted into naphtha but you get less naphtha than input diesel
That's basically the only thing you can do with diesel outside of actually using it for a process
U can also either

So I guess the chain is actually
heavy -> naphtha -> fuel gas, with diesel as an alternative to the fuel gas step, and the gas->diesel recipe is just to make all the digester recipes actually useful earlier

In terms of energy I think cracking heavy to naphtha and burning naphtha and fuel gas separately, (+ reforming some fuel gas into hydrogen for cracking) is the best, return, I think
Am I getting that right?