Rather than using gear inches, think in terms of percentages. An 8-11% jump is most common. As the cogs get larger, the percentage jump decreases, intil a 1T jump becomes too small to be worthwhile. The 70-50 inch range that you've mentioned is a 42T chainring with cogs ranging from 16 to 23 teeth. You can get a cassette with a 1 tooth jump from 16 to 19, but the 20T cog is rarely found for a very good reason - the percentage jump is just too small to be meaningful. A junior cassette will have it, just because it's needed for filler material.