I think you are referring to this insert that says use 150 M if the scale goes to 150 and use 170 M if your scale goes to 170. What they really mean is use Max M and take what you get.

http://www.ilfordphoto.com/Webfiles/...8932591755.pdf

To really get Grade 5, I think you need to add 50 M or 70 M as a filter in the filter drawer (assume your colorhead has one, CLS35 does, and instructions say if you need more density use the filter drawer.