Thanks for the comment! The difficulty of the pattern determines the max points you can get. Yin Yang is one of the hardest but pays out well if you get it right :)
It works off taking a screenshot of the screen at the end of the timer, the pixel colours are comapred between the reference image and the nail with the relevant scaling factor. Each pixel in the nail is checked and the percentage accuracy is multiplied by the difficulty of the design. Hope that helped :) If you want more info, the code is very messy but I could upload it to github if needed.