It's not quite like that. There is some overlap between the ratio's, it's not a linear progression. SAy you are in 4th on the rear, large on the front, you feel like peddling is a bit too slow but 3rd feels too fast. Then the answer may be to go to small front cog and 5th on the rear. Sounds complex, I know, but it's all a matter of feel. Come to a slight uphill, change front cog from large to small. Headwind on the flat - small cog. Downhill, on the flat or tailwind, large cog, then select rear cog as required.
I live in a hilly area and have a narrow cadence range that I feel comfortable with, so I am always changing gear - both front and back. Others will say they rarely change the front cogs and even ditch them entirely.