In the example we looked at with the various pens, it wasn't necessary to measure Rf values because you are making a direct comparison just by looking at the chromatogram.
You are making the assumption that if you have two spots in the final chromatogram which are the same colour and have travelled the same distance up the paper, they are most likely the same compound. It isn't necessarily true of course - you could have two similarly coloured compounds with very similar Rf values. We'll look at how you can get around that problem further down the page.