Payload rate to hide text in image?

I'm working on a method to detect the hidden information in images and used some tools to hide text inside the image but the problem which faced the tool don't have a choice to determine the payload rate such as 0.02, 0.10, 0.20, 0.30, 0.40, and 0.50 bpp. what is the length of the text characters corresponding to these payload rates when I want to represents them as a messages that I will hide in the image.

Thank for your answers