How do you know which gear to use?

Hi all,

I have owned my brompton for a month now and have been cycling in general for about 3 months now. I've ridden over 1000km and I still don't know when to use what gear for best efficiency.

For example, I have a 6-speed C line. I use gear 5 for flatland, 4 for slight incline, 3 for bridges, 1&2 for mountains and 6 for declines. Is this a good way to use the gears, or is there a better way to measure when to use which gear?