The idea of drawing up the quota is: 20mm 1 for artificial gravel paving is set as X, 1 for adding 1 is set as Y, and 30mm is set as X+ 10Y. The thickness of X in the quota is generally the minimum value specified in the specification. I use it for decoration, such as 20mm for cement mortar leveling. We can only increase it but not decrease it. Your 8mm screenshot is X- 12Y, and the compilation of y only considers increase, not decrease ... so there is a negative number. Is the drawing 8mm? Is this up to standard? If possible, the suggestion is 8Y sets, but this should make the price lower. Do you understand this writing? Welcome questions.