Great question! I personally believe in intuitive eating, and wrote a bit more extensively behind this idea here on "harahachi-bunme". But basically, if you have a higher metabolism and you are hungry, you should eat! Everyone's "balance" looks different, and this will change over the course of your own lifetime as you age and change your lifestyle 😊
And with regards to brown vs. white rice, I think the key is not fussing too much over the small details with every meal when thinking about leading a healthy lifestyle, but focusing more on moderation and making sure we are getting the variety we need in our diet over the long term.
Nutritionally speaking, brown rice is higher in fiber and nutrients, so it would be great for one to eat brown rice more often than white! But as you pointed out, a lot of individuals eat white rice with almost every meal yet lead very healthy lifestyles. I think it's because these cultures tend to balance white rice with other fiber and nutrient-rich foods, and consume it in moderation-- rarely is a meal just a big bowl of white rice with soy sauce-- which makes the choice of white vs. brown rice not have such a big on our overall health.
Hope this helps! And feel free to ask more questions at my email firstname.lastname@example.org 😊 Best, Kaki