Try printing out a color chart on the shirt and reverse matching from there.
If you are getting purple you have to much magenta in the print. Look at getting a Pantone to process swatch book. This will help illustrate how close you can get to a formulated solid with four color process.I am working on printing out t-shirts that will match the royal blue ringer. I have tried 6 different setting to try to get a CMYK match or near it. Currently it is printing out purple.