Often the % of palisade is related to clolorophyll content per unit area. In order to calibrate it you need some trasverse section and stereology as in a free thesis or by D Parkhurst papers in Jstor; the thesis of S. Short Russel is  at https://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=17&ved=0ahUKEwj2jp2o_L_MAhXJAcAKHX1oBWc4ChAWCDkwBg&url=https%3A%2F%2Frepositories.tdl.org%2Fttu-ir%2Fbitstream%2Fhandle%2F2346%2F17041%2F31295004922463.pdf%3Fsequence%3D1&usg=AFQjCNFUcUgOq3vBBIYLjF1A6ReOpD-EaA&sig2=5LrYtlJiCzsD3of9J1ydhA&cad=rja

Thank You Sir@Guido Bongi

You can do the cross section of the leaf segment, take the photo under microscope (do not forget to add scale) and measure various parameters (length, width,cell area...) in software such as ImageJ or Lucia and others.