I would think that working backwards from required brine volume...

I would think that working backwards from required brine volume is the starting point.
If you know the grade and overall recovery rate then you can work out how much brine you need to process to achieve the signature output.
Once you know the brine volume required to achieve signature output you can look at trench requirements which requires knowing trench flows.
These guys have increased their signature output from 370ktpa to 426ktpa and claim to have a 80% recovery, an increase from 69% in the scoping study. The Trench requirements have gone from 250km to 550km. Signature output has increased by 15%, recovery has increased by 14% and trench has increased by over 100%. Note that pond size has stayed the same from the scoping study. When considering the ponds it's worth noting that the scoping study was based on dry harvest as per RWD.
The trench increase could just be insurance and will probably be sold as such, trench is cheap etc we have the most real estate blah, blah, blah. Secondly it could be that the final trench flows aren't quite as flash as the initial reports. Thirdly, the original assumptions were way off like, for example, the process water requirements.
Note that RWD is now laying around twice as much trench as initially suggested following the first round of trench trials.