Hey, Firstly you have two measure two things with your Ruler A) the distance moved by the substance from the line which we drew with pencil in our practical and B) the distance moved by the the solvent from the "pencil" line this is basically from the pencil line to the line at the top which I believe is allways labelled.
Now the easy part we divide part A's answer by part B's answer. So the rf formula is the distance moved by the substance/ the distace moved by the solvent. The subsatnce is the marked by the dits and if there are more thean one dots then it says which dot it wants you to measure. I advise you measure the Middle of the dot as well.