You should increase your bubbles slowly and measure the Co2 levels with each change.
30bpm is giving you 6ppm Co2.
Increase to 40bpm and then recalculate your ppm of Co2 to see the change.
If your change was very small, then you can increase the bubble rate by more than 10bpm the next time, maybe by 15bpm increments or more.
If your new ppm is still too low, continue increasing the bpm by increments, measure the changes, etc. until you hit your Co2 target.
Bubbles per minute is only useful to you because it allows you to visually confirm that the amount of Co2 you are injecting isn't changing. For example, if I walk by my reactor and notice that it's firing 2 bubbles per second then I know my needle valve must have slipped or failed because I haven't been injecting that much.
However, if I set up a new tank of a different size or with a different reactor or water source I couldn't assume that injecting the same bpm into it would yield the same Co2 as in my original tank, because water chem, volume, plant load, lighting levels, diffusion efficiency etc determine how much Co2 has to be added to get the levels in the water to a certain ppm.
It's like looking out a window of a car. You can't tell your mph by how fast the grass wizzes by. But if you notice how fast the grass wizzes by when you are going 50mph, then you can determine that you are going slower than 50 if the same grass wizzes by more slowly. But it still doesn't let you know your new speed. Are you now going 45mph or 38 mph? It is only helpful in a relative manner.
"Insanity: doing [or asking] the same thing over and over again and expecting different results."