to the 30 yard line didn't take place until the early 2010s. So you are looking at probably a smaller sample size of kicking off from the 30 yard line and no faircatch inside the 20 yard line compared to the other scenarios:

1. Kickoff from the 40 yard line with a touchback starting at the 20 yard line.

2. Kickoff from the 35 yard line with a touchback starting at the 20 yard line.

3. Kickoff from the 35 yard line with a touchback starting at the 25 yard line.

So from a statistical standpoint, even though it may appear to be more difficult to kick the ball out through the end zone from the 30 yard line (the only guarantee way to get a touchback), the kickoff-touchback ratio may not be that much different for all 4 scenarios.