I often see in papers that the % consumed leaf area is visually estimated in classes, mostly 20% steps: class 1: 0%, class2: 1-20%, class 3: 21-40% ...
For some pests and plants there are scoring table, e.g. for Psylliodes chrysocephala feeding on oilseed rape, see here: https://images.app.goo.gl/9uKuxcXuXnYoQDfa6
You can also make pictures and the use software to measure the consumed leaf are, e.g. with the free app BioLeaf or with the computer programm ImageJ.
I think all of my mentioned approaches are quite standard.