0-255 to % Converter is a calculator for 3ds Max from Jon Seagull that converts percentage values to 0-255 values and back. It handles the translation in either direction, and it's listed here as free.
Alongside the conversion, it includes a color swatch that updates with the grayscale value equivalent. The script is at version 1 and lists compatibility with 3ds Max 7, 6 and 5. A demo video is available, and Jon Seagull's page carries further details and links.
As a consultant and technical director, Jon Seagull works on the technical aspects of clients' internal 3D pipelines. That work can include technical direction for large or unusual projects, custom tool creation and scripting, emergency troubleshooting, or over-the-shoulder best practices instruction.