Voila la solution
(en anglais)
This is known usability issue. When you finish editing your assessment form, save and continue editing it. Note there is yet another form at the bottom of screen. Here, you have to define the mapping table - that is what number of errors leads to what grade (in percents). The numbers displayed beside the drop-down menus are a hint. If you use them (select them manually in drop-down menus), you will have linear conversion.
This user interface, besides many others in the Workshop, deservers some usability improvement to make it more intuitive, for sure.
Hope this helps.