It was indeed all about the roundabouts. I forget the details but I do know it took us quite a while to get there(!)
Those tutorials filled me with dread at the time, but with hindsight they were - how can I put this - a fairly formative experience.
Watching your tutor use paper and pencil - and estimation - to calculate something like that was actually quite inspiring. That was, once it stopped being terrifying.
Presumably without roundabouts it's all random directions and balanced?
Or am I barking up the wrong tree?