Just curious, why would you need to select only a percentage of a pantone color?
I had 6 retractables to do and the file had some of the graphics set at 80% of the pantone colour of the rest of the banner. The file was not set up with the proper colours so I had to make the changes before printing.
I wonder though with the way these printers work and profiles if simply adjusting the %age of any colour actually gives the desired results.
Take for instance black...if you were to go into the cmyk settings for black and hypothetically said I want a 50% black and then re input 50% of all the values for each colour I am pretty sure the colour you will get is anything but grey or at least nit a true 50% black. does that make sense?