I’ve learnt my pots are too big, so what size pot for this Zanzibar plant/ where do I measure the root ball from?

On another post I made here recently I learnt a lot of my plants are in pots too big (resulting in fungus gnats) which I didn’t even realise. I was more so taking into consideration how big the plant might grow to.

So for this plant as an example how do I judge what size pot I need? I’ve been told to get a pot 1-2inches bigger than the root ball. In the last picture I’ve drawn some red lines where I think I’m meant to measure the 1-2inches from, is this correct? How much do I have to take the height of the roots into consideration? I know this pot is too deep but I’m worried my next one might be too shallow